I took my 13' GT w/ 3:73 gears in for a whine in the rear end. It starts around 60mph and increases with more throttle, but goes away when lifting off the throttle. The dealer found that the rear diff was 1 QT. low on fluid from the factory. Car only has 8800 miles on it. No leaks were found.

I'm concerned about the longevity since it obviously wasn't filled properly from the factory. Dealer said to drive the car for a while and come back if it's still noisy.

 

Drove over 200 miles and the noise hasn't changed. Guess I'll be taking it back in.

 

Is it a high pitch oscillating noise thay starts around 64 and goes to 80mph?

Mine sounds similar but I believe it is coming from the front.

 

It's not too high pitched, but it's definitely coming from the rear.