Bearing Whine
Bearing whine is a high-pitched sound similar to a whistle. It is usually caused by worn/damaged pinion bearings, which are operating at driveshaft speed. Bearing noise occurs at all driving speeds. This distinguishes it from gear whine which usually comes and goes as speed changes.
As noted, pinion bearings make a high-pitched, whistling noise, usually at all speeds. If, however, there is only one pinion bearing that is worn/damaged, the noise may vary in different driving phases. Do not install new pinion bearings unless bearings are scored or damaged or there is a specific pinion bearing noise. A worn/damaged bearing will normally be obvious at disassembly. Examine the large end of the rollers for wear. If the pinion bearings original blend radius has worn to a sharp edge, install a new pinion bearing.
A wheel bearing noise can be mistaken for a pinion bearing noise. Check the wheel bearing for a spalled cup, and spalled/damaged rollers. Install a new wheel bearing and cup if any of these concerns are detected.