Diff. Question

[A666K@aol.com]

Hey all,
A friend asked me to forward a question; 
His '72 TR6 developed a horrible differential noise. It is not a clicking 
sound that I am familiar with, but a terrible shriek/whine when he is at hwy. 
speed, under load. This can be heard two lanes over, which is why it is 
annoying me as well. So far he has made sure the lubricant is at the right 
level, nothing else. Any clues or ideas on what to look for? 
TIA, 
Avi