I have a 1984 chevrolet Pickup and just had the entire ac system overhauled. System was evacuated and filled with 134a.
Parts changed:

Compressor
Hoses
Dryer
condenser
oriface tube
Idle stop solenoid

After the system was charged and the Idle stop solenoid was set (while the engine was in park) the compressor didn't make any noise. I do however have to tap the gas after I turn on the AC to get the Idle to adjust to the proper setting on the Idle stop solenoid. If I don't tap on the gas the clutch in the compressor knocks. The other problem is when I put the vehicle into drive and hold my foot on the brake and let it idle the compressor starts to knock again. (BTW this is the second new compressor installed in 3 weeks). I have a feeling the RPM's are not getting high enough to keep the compressor functioning properly with the load it's carrying even though it is blowing about 60 degree F air out of the vents.

The other problem with my truck is it sounds like the catalytic converter is out or going out and I have no mufflers on the vehicle. I have been told that my truck needs some exhaust compression to function normally. So my question is, is my converter causing my engine/ac to not function properly due to the free flow of the exhaust through the exhaust system or is my ac problem another fault?