A/C probs...????
 

Okay, installed new compressor and drier the other day. Now at idle in Park it makes a very faint noise. When at idle in gear it makes a racket. When the A/C is off it makes no noise. With A/C on going down the highway it makes faint noise. Should I have hooked everything up and then evacuated the system before adding R134a?

No, evacuting and refill of your freon is not going to help a thing with your noises.

The noise that you hear could be very well come from the compressor itself, due to bad rebuild.

When your a/c is kicked in it also put a big load on the belt and from here it brings out the best of all your old and fatigue parts like, belt damper, belt tensioner, belt tensioner pulley, alternator, power steering pump, etc...
I would check if the compressor is leaking oil, then replace the belt damper( the little shock absorber) also check and adjust the belt tensioner.

Alex