Thinking about buying a 91 Caprice from a friend. It's a solid vehicle, but it makes an awful knocking type sound when you turn the A/C or heat on. It purrs like a kitten if they are off. Any ideas? Thanks in advance.

 

Post a video of it.

 

When you turn the heat on ? Look & see if the compressor is rotating / functing when the heat is on ( not just the compressor pulley , but the clutch is rotating also ) .

Check the belt powering the compressor pulley .

You sure the noise is not from the inside climate control fan ?

God bless
Wyr