If you’ve ever heard a whining, humming sound coming from the front of your vehicle once you engage 4WD, then this article is for you. We will look at the possible components that could cause the whine and why you only hear it in 4WD mode and not in 2H (Two-Wheel Drive)
Why does my 4-Wheel Drive whine? In most cases the problem can be one of the following components:
- Front differential Gears
- Differential fluid low
- Driveshaft U-joints.
- Transfer case wear
- TC’s low oil levels
- The front Vacuum hub assembly leaking
Any one of the above-mentioned components could be responsible for emitting an audible humming or whining noise when you engage in 4WD mode. Other reasons could be mismatched front and rear tire diameter, low transmission fluid, or worn wheel bearings, however, these will whine and hum in 2H mode as well.
Next, we’ll diagnose the problems further and cover the possible causes and downstream components that could possibly be affected, if any.
You might want to read any of the below articles if your 4-wheel drive is emitting any of the below symptoms:
- Your 4-wheel drive makes a clicking sound – Click here.
- Your 4-Wheel drive makes a popping sound – Click here
- Your 4-wheel drive makes a grinding sound – Click here
- Your 4-wheel drive makes a clunking sound – Click here
Table of Contents
4-Wheel Drive Humming Noise
So far we established that the humming noise is only prevalent once 4WD is engaged and not while driving in 2H. This is due to the fact that 2H mode only rotates the rear driveshaft or prop-shaft as some call it and does not engage the front driveshaft, differential, and transfer case at all. So we can safely omit those when in 2H mode.
Whining in 2H (Rear Wheel Drive)
If you are in 2H and you still hear a humming noise it’s more than likely one of three things. Those are any of the following components:
- Rear differential low oil
- Wear on the Rear differential internal gears
- Wheel bearings (Front or rear)
- Transmission wear
- Low Transmission oil levels
These are only applicable when selecting 2H mode, which is what you will be in most of the time since it’s designed for high-traction surfaces such as tarmac, concrete, and highways.
Let’s look at the possible culprits in more detail now
Whining Noise in 4WD (Front Differential Gears)
The front differential function is to allow the transfer of power to each wheel in front. When the 4-wheel drive is engaged the front differential splits the 50 percent of the power it receives between each wheel. The most power will be sent to the wheel with the least resistance and the differential achieves this by means of a ring gear, pinion gear, and a set of spider gears all mounted inside a carrier. These are all securely fitted inside the differential and set at micro-millimeters of an inch and perfectly balanced.
All these components are high-stress components and huge amounts of torque are transferred through them each time you drive in 4WD. All the components inside the differential should always be surrounded by the correct level of oil to lubricate and keep operating temperatures low to avoid premature failure.
If the oil levels are low inside the differential the high levels of torque being transferred through it place more stress on the metal resulting in the current oil heating up higher than its viscosity can handle.
This, in turn, begins to break down the oil viscosity further resulting in the metal components not being protected and lubricated sufficiently. This is the cause of metal being worn off quickly and gear teeth becoming rounded creating excessive movement inside the differential. The combination of low oil levels and wear on differential gears results in an audible whining sound when 4WD drive is engaged.
Begin by checking the oil levels in the diffs and for any fine metal filings in the oil. If none are found but the oil is very thin, watery, and black, it could simply indicate the oil was never changed regularly and needs to be drained and replaced with the correct differential oil as specified by the manufacturer.
Whining Noise in 4WD (Differential Fluid)
As previously mentioned, low differential oil or even old differential oil could be the cause of the differential emitting a loud whining or humming noise when 4WD is selected. Diff oil is a lot thicker than engine oil and is specifically designed to withstand high pressure and temperature change.
Make sure you check your owner’s manual for the correct specification of differential oil for your specific vehicle. Ensure the oil you use protects against all the necessary friction for most synchronizers of mechanical gearboxes. Remember, automatic transmission oil is different from manual transmission oil and cannot be interchanged.
Whining Noise in 4WD (Driveshaft U-Joints)
These components are wear and tear items and need to be regularly inspected. They are situated at almost the lowest part of the 4-wheel drive and are very susceptible to moisture, heat damage, and knocks when driving off-road. Before and after each off-road trip, crawl underneath your 4-wheel drive and have a look around or drive it to the nearest suspension and drivetrain garage. Put your 4-wheel drive on a hoist and have a professional inspect the drivetrain for any damage or leaks.
The next extract is from an article you can read here and describes the function of the u-joint.
The Universal joints are located at the ends of the drive-shafts. The rear U-joint connects directly to the rear differential and the front end of that same driveshaft is connected directly to the transfer case. The front section of the front driveshaft connects directly onto the front axle via the differential. Vehicles like Jeeps with solid front axle suspensions also incorporates U-joints on each front wheel.
The U-Joint allows the drive-shaft to connect to the diff at an angle and compensates for vertical and lateral movement of the differential while the vehicle is in motion.
If you are driving in 2H, which you should be if you own a part-time 4-wheel drive. This mode only engages the rear driveshaft and propels the rear wheels only. In this 2H mode, the front driveshaft in inactive as well as the front differential and all u-joints forward of the transfer case.
extract taken from article titled: Why Does My 4 Wheel Drive Clunk
If the u-joint rubber caps get damaged or wear out, allowing dirt and debris into the needle bearings inside the u-joint, it will cause the u-joint to fail. Also if the rubber seal perishes due to old age and all the lubricating grease runs out, the u-joint will fail.
Whining Noise in 4WD (Transfer Case)
The function of the transfer case is to lock the front and rear driveshafts and send equal power to both front and rear axles via the differentials. There is an extremely high level of torque that gets transferred through this component and if the driver does not exercise caution when engaging it, he will break something expensive. The transfer case is only designed to be operated under low traction surfaces such as:
- Muddy tracks
- Sand dunes
- Icy, slick roads
- Slippery inclines or declines off-road
Pretty much any boggy, greasy, slick, or loose surface is ideal for locking the center differential. There has to be a small level of wheel slip in order for it to function correctly without failure.
If the transfer case is engaged under high traction surfaces such as driving on the highway or on dry pavement, you seriously risk transmission windup or drivetrain binding.
You can read more about that here
A whining sound being emitted from the front of your 4-wheel drive only when 4H or low is selected could mean there has been some damage inside the transfer case to the gears. Once drivetrain or transmission windup occurs, there are massive amounts of torque that are redirected back towards the TC and this places excessive wear on metal gears inside resulting in damage and giving off a winding sound.
Below is an extract about transmission windup you can read more here.
4 Wheel Drive Locks Up When Turning
This effect causes your vehicle to under-steer heavily, gears to get jammed and makes steering very difficult and even jerky. This phenomenon is caused by the front wheels battling the rotational force coming from the front drive shaft as it tries to slow down the front wheels, causing the massive under-steer effect. You should avoid engaging 4WD on a high traction surface at all costs. The longer you drive in that mode, you risk serious damage to your drive-train components and you will find it increasingly difficult to remove it from 4WD mode and switch back to 2WD mode.
4WD Binding – How To Fix Transmission Wind-Up
If you have forgotten to take your 4WD out of 4H after heading back on to the tarmac you will definitely experience transmission wind-up after a while. One way to identify if you have transmission windup is, your vehicle being stuck in a gear. You can yank and hang on the gear lever but nothing will release that gear due to the immense forces and torque built up inside the transmission.
So how do you fix transmission wind-up? The easiest way to fix bind-up is by pulling over to the side of the road with two wheels firmly on the tarmac and the other 2 wheels on a slippery surface like grass, mud, snow or sand. This allows the wheels to rotate at altered speeds releasing the wind-up in the transmission box. Once the wind-up has been released you will then be able to use your gearbox/transmission properly again.
The other option is to reverse in the same direction you were driving. So if you were driving forward in a left direction, reverse in a left direction and allow the wind-up in the transmission to reverse itself naturally.
Extract taken from the article: Why Your 4 Wheel Drive Jerks and Feels Hard to turn in 4WD
Whining noise in 4WD (Transfer Case Low Oil)
Another reason for whining sounds only when 4WD is selected is the low level of oil inside the transfer case. Old degraded oil is also a reason for whining sounds coming from below the 4-wheel drive in 4H. This should be inspected and replaced if necessary.
Before you drain the transfer case oil first refer to your owner’s manual for specific fluid change recommendations and exact TC oil specifications. The transfer case fluid should be changed every 30,000 miles especially if you use the four-wheel-drive off-road often.
Whining noise in 4WD (Front Vacuum Hub Assembly Leak)
If you drive a 4-wheel drive with manual front hubs, you could experience a humming or whining sound coming from the front once 4WD is selected. This could be caused by a cracked or perished rubber seal inside the hub assembly. Because it works with air pressure, also inspect the rubber lines for any cracks or breaks.
The entire wheel assembly needs to be disassembled including the lower control arm and brake calipers on certain models of 4WD’s. Inspect all the ball joints while everything is loose as well.
Front-End Noise in 4WD
Other noises that could possibly be causing humming and whining noises from the front end are:
- If you’ve recently replaced or rotated the front tires and they are direction-specific tires.
- Also, if the front and the rear wheels are not the same diameter. The difference in the rolling circumference could result in a whining noise when 4-wheel drive is activated.
Worth a watch
Conclusion
So we’ve established a few possibilities that could cause humming or whining sounds only when 4WD is active. Dry components and joints are your first point to inspect. Also, take into account any irreversible damage that could be caused due to mistreatment or negligence. Transfer cases and transmission boxes whining are common side effects of 4-wheel drives that have towed heavy boats, trailers, or caravans for extended periods. Unfortunately, replacing the oil might lessen the whine somewhat but not eradicate it completely.
Transmission windup is a phenomenon that causes serious damage to internal components inside the transfer case and differentials. This can result in teeth breaking off inside the differential and metal filings or pieces moving around inside the differential.