When road testing a vehicle, a smooth asphalt road surface is preferable because it dampens a high degree of road noise. Road test the vehicle to verify the problem and the speed and/or rpm at which it occurs. Once the problem has been verified, duplicate the noise under varying road conditions to eliminate the possibility of road noise. Once the noise and the conditions under which it exists have been verified, the type of noise and its location must be identified.

Many noises thought to be coming from the front axle may actually be originating from other sources. These may include:

Because noise is a major factor in determining or diagnosing a front axle problem, a thorough and careful check should be made to determine the source of the noise before proceeding with front axle repair.

Some road surfaces such as brick or rough surfaced concrete cause noise which may be mistaken for tire or front axle noise. Driving the vehicle on varying types of road surfaces such as asphalt or dirt will help in determining whether road surface is the cause of the noise. Noise caused by road surfaces will be the same in drive, coast, two-wheel or four-wheel drive.

Tire noise can easily be mistaken for front axle noise. Because noise can transmit or dissipate through the vehicle body, noise coming from the front tires can also be mistaken for a front axle noise. Tires showing uneven wear or under-inflation are usually noisy and may produce vibrations which can appear at other places on the vehicle. This is particularly true with low tire pressure. For tire inspection procedures, refer to Tire Diagnosis - Irregular or Premature Wear .

The engine or transmission may sometimes produce noises which may be mistaken for front axle noise. To determine which unit is actually causing the noise, record the speed, rpm and other conditions during road testing, at which the noise is most pronounced. Then stop the vehicle in a quiet place to avoid interference with other extraneous noises. With the transmission in neutral, slowly run the engine speed up and down to the engine speeds that correspond with the vehicle speed at which the noise is most pronounced. If the noise is produced with the vehicle at a standstill, the problem lies in the engine or transmission and not the front axle.

Brake noises caused by the front or rear brakes are rotational once every wheel revolution. During a road test, duplicate the conditions under which the noise is most pronounced and gently apply the brakes. If the noise immediately appears or disappears when lightly depressing the brake pedal, the noise may be caused by an out of round brake rotor or drum. For brake diagnostic and repair procedures, refer to Brake System Testing .

Objectionable body boom noise or vibration usually occurs at 55 to 65 mph (90 to 100 km/h) and can be caused by an out of balance propeller shaft. Excessive looseness or wear at the propeller shaft sliding yoke splines may also cause a propeller shaft to be unbalanced. If a propeller shaft is suspected of being out of balance, refer to Correcting Driveline Vibration - RWD and 4WD Vibration, Diagnosis and Correction for propeller shaft inspection procedures.

If all indications point to a front axle noise after making a comprehensive check of the vehicle, it is necessary to determine at what component of the front axle the noise is occurring. Front axle noises usually occur at one of two places: drive axle shafts and differential gear or bearings. Refer to Wheel Drive Shafts in order to determine the possible causes of front drive axle shaft conditions.

Side gears and differential pinion gears rarely cause noise. This is due mainly to their lack of movement during straight ahead driving. Noise produced by these gears will be most pronounced on turns.

Pinion bearing failures can be distinguished because pinion bearings rotate at a higher rate of speed than the differential side bearings or drive axles. Rough or excessively worn pinion bearings produce a continuous low pitched whirring or scraping noise on low speed acceleration from a stop.

Side bearings, when worn, produce a constant rough noise at a lower pitch than with pinion bearing noise. Side bearing noise may also fluctuate with drive axle shaft noise.

There are two types of gear noise. One type is produced by broken, bent, overheated or forcibly damaged gear teeth. This noise is usually quite audible through all speed ranges and can easily be diagnosed upon visual inspection. For example, hypoid gear tooth scoring generally results from the following:

The scoring will progressively lead to complete erosion of the gear teeth and, eventually, a fracture will result if the initial scoring condition is not corrected. Another common cause of hypoid gear tooth fracturing is extended overloading or shock loading--causing the differential pinion gears and shaft to seize. This can result from lubrication breakdown caused by excessive wheel spin. The second type of gear noise pertains to the contact pattern of the ring and pinion gear teeth. This form of abnormal gear noise can be recognized because it produces a cycling pitch (whine). This whine will be very pronounced at the speed range in which it occurs. Because the front differential is driven (powered) in four-wheel drive only, this noise can only be duplicated in four-wheel drive. Gear noises in the front differential can occur under one or all of the following conditions:

Gear noises tend to peak at a narrow speed range or ranges and tend to remain constant in pitch. Bearing noises will vary in pitch with vehicle speeds.