Front Axle Noise Source Determination

Road Noise
Some road surfaces cause noise that may be mistaken for tire or axle noise. Road noise is usually the same while driving or coasting. Driving on a smooth surface helps determine the cause of the noise.

Perform the following maintenance/diagnostic inspections:
^ Inspect the tires for irregular wear.
^ Test the tire pressure.
^ Inspect the front axle lubricant level.
^ Drive the vehicle in order to warm up the front axle.
^ Drive the vehicle at various speeds in drive, float, coast, and while cornering.

Tire Noise
Tire noise continues with a lower tone as the vehicle slows down. Tire noise may easily be mistaken for axle noise. Tire noise changes with varying road surfaces. Axle noise does not change, although, the axle noise usually stops when coasting at speeds under 48 km/h (30 mph). Temporarily inflating all the tires to 345 kPa (50 psi) pressure for test purposes only will materially alter the noise caused by the tires, but will not affect noise caused by the axle.

Perform the following maintenance/diagnostic inspections:
^ Change the tire pressure in order to minimize noises.
^ Drive over different road surfaces.
^ Drive on smooth blacktop in order to minimize tire noise.
^ Cross switch the tires, if necessary.
^ Remember that snow tire treads and tire studs cause added noise.


Engine and Transmission Noises
In order to determine which unit is at fault, take note of the approximate vehicle speeds and the conditions under which the noise is most pronounced. Stop the vehicle in a quiet place in order to stop interfering noises. With the transmission in neutral, run the engine slowly throughout the range corresponding to the vehicle speed at which the noise was most pronounced. If a similar noise is produced, the noise is caused by the engine or transmission and not the axle.

Perform the following diagnostic inspections:
1. Drive slightly above the speed where the noise occurs. Place the transmission in neutral.
2. Let the engine speed drop to idle.
3. Stop the vehicle.
4. Run the engine at various speeds.

Wheel Bearing Noise
Raise the vehicle on a twin post hoist. Spin the wheels by hand. Listen for a knock or a click about every 2 revolutions of the wheel. The bearing rollers do not travel at the same speed as the axle and the wheel.

Perform the following diagnostic inspections:
1. Drive the vehicle at low speed on a smooth road.
2. Turn to the left and turn to the right. The noise should change with the cornering loads.
3. Jack up the wheels in order to verify a roughness at the wheels.

Gear Noise
There are 2 basic types of gear noise. The first type is produced by broken, bent, or forcibly damaged gear teeth. This type is usually quite audible over the entire speed range, and presents no difficulty in diagnosis. Hypoid gear tooth scoring is 1 example that generally results from the following conditions:
^ Insufficient lubricant
^ Improper lubricant
^ Improper break-in
^ Insufficient gear backlash
^ Improper ring and pinion alignment
^ Loss of drive pinion nut torque

The scoring will progressively lead to a complete erosion of the gear teeth or gear teeth pitting. Then there is an eventual fracture if you do not correct the initial scoring. Another cause of hypoid gear fracture is extended overloading of the gear set. Extended overloading will produce fatigue fracture, or shock loading, which will result in sudden failure.

The second type of gear noise pertains to the mesh pattern of the gear teeth. You can recognize this form of abnormal gear noise because the noise produces a cycling pitch (whine). The whine will be very pronounced. The whine tends to peak in a narrow speed range, appearing under drive, float, or coast conditions. The whine will remain constant in pitch. Bearing noise will vary in pitch with the vehicle speeds.
^ Rough running or a whine noise should increase with speed.
^ Noise pitch should be higher than differential noise.
^ Front pinion bearing noise may be louder on deceleration.
^ Rear pinion bearing noise may be louder on acceleration.

Perform the following diagnostic inspection:
^ Road test the vehicle on a smooth road in order to minimize tire noise.
^ Road test the vehicle at various speeds in drive, float, and coast.