I have a 1985 Z28. The AC blows very cold but I hear a load vibration in the car when I turn it on. Upon inspection the AC belt runs fine when the AC is off but once I turn it on the belt jumps up and down quite a bit and hits the compressor bracket which is causing the noise. Belt is not loose. Anyone have this issue?

 

My aunt had a '78 Cutlass that did that-same R4 compressor on a 260 Olds.Check that the upper bracket support strut is in place.Also could try a different AC belt-new and as short as will fit. 88-91 Corvettes had a disc on the WP pulley to dampen belt vibration with AC on

 

These cars all have a tendency to do that. It's the compressor mount system flexing as the pulses of compression are produced. It's almost always worst at idle or close to it butt smooths out at higher RPMs.

Some came with an extra bracket, that goes from the top of the intake to the top outer edge of the big bracket that goes over the top of the compressor. It helps quite a bit butt sometimes even that doesn't cure it. It's possible to make an extra brace to go to an exhaust bolt, which is more effective.

 

Swapping to a serpentine belt system solved that problem for me