Rear End Noise

[hevy_chevy67]

Yesterday I swapped out my entire rear from the narrower 67-70 style to the 71-72 style. I bought the replacement one from a guy parting out a project and it had been gone through. 3.73 posi, everything looked good to me, but I'm not expert haha.

Anyways, it's making a whinning noise once I get some speed going. Below 30 mph everything sounds fine, and even on acceleration and when I let off the gas the sound is completely gone or at least nothing that I can hear. It seems to rear up when just feather the gas or when under light load. Any ideas? I'm going to go get it checked out but it's pissing me off. Driveshaft was recently balanced and no such noise existed with the old set up in there.