The most essential part of axle service, as with any mechanical repair, is proper diagnosis of the problem. In axle work, one of the most difficult areas to diagnose is noise. Because the front drive axle shafts and differential turn in four-wheel drive only, all road tests and checks should be performed in and out of four-wheel drive to eliminate the possibility of the problem originating in the transfer case or rear axle. Because the front drive axle shafts are flexible assemblies, most failures occurring with a drive axle shaft will produce a noise with a rotational characteristic. In other words, a noise that is pronounced with a click or knock on every drive axle shaft rotation. These noises can usually be distinguished from differential noises because they can occur at lower speeds or around turns. Differential gear or bearing noises normally occur at high speeds or at one specific speed. Any gear driven gear unit, especially a gear driven axle where engine torque occurs at a 90 degree angle, produces a certain amount of noise. Therefore, an interpretation should be made for each vehicle to determine whether a noise is normal or if a problem actually exists. One way of determining this is road testing an identical vehicle with the same options and under the same conditions. If the same amount of noise is audible at the same speed and under the same conditions, it must be deemed commercially acceptable since it is unlikely that the exact same problem exists in both vehicles. Therefore, a normal amount of noise must be expected and cannot be eliminated by conventional repairs or adjustments. However, if a noise proves to be extremely loud or unusual, it should be investigated. This can be accomplished by a road test and a systematic check of the components associated with the front axle to isolate the noise.