While driving my vette back to school yesterday I found a new noise... It seems that when I am in 4th gear between 2800 and 3000 RPMs there is a "grinding sound". It sounds like it is coming from the rear end. However it doesn't make the sound in any other gear. I have noticed I clunk sometimes when starting out in first gear. Because of this I checked the U-joints and they seem fine? Do I need to get my rear end re-built? and if so how much is that going to set me back? P.S. I think my transmission is an M-21 or a M-22... thats what the last mechanic told me when we fixed my linkage.