when i put my 91 Z28 in reverse it first is fine, then i let off the brakes and its still fine, but if i press the brake while reversing it makes a loud anoying buzzing/vibrating noise like the noise brakes make when they are holding the car from moving but not a complete grip. my brakes are fine. and when i stop completely while in reverse it the noise continues. my first thoughts were a back up sensor telling my im gonna hit something but i dont know what those sound like and i wasnt near anything anyways.
this is all so confusing to explain, any ideas?

 

It could be worn reverse friction plates. Usually reverse has the least number of friction disks. Any reverse brunouts lately?

 

i havnt done any reverse burnouts. bought car not too long ago from a guy that had it off the road for 2 years.
if this is the problem any estimate on how much it might cost to get repaired?

 

It would cost between 3000 and 1000 to do a rebuild. Have you checked your trans fluid? What color is it?