Howlin' Diff
 

Race car diff gets to howling when hot (20 minutes or so at speed).

Cause for concern? My inclination would be to ignore.

Try changing out the gear lube, and make sure it's topped off...The sounds of howling from the Transaxle is not good...hopefully it's just the lube that can be changed or it's just low - Howling like that could be a sound of bearings going south...