a/c noise at stoplight in drive
 

Air Conditioner making noises
I have a 1986 Mercedes 190e. As far as the a/c working. Works Great. But when I come to a stoplight and stopped in drive I get a rumbling / knocking type sound. Now at the stop light if I put the car in neutral. RPM raises alittle the noise lessens. If the RPM is 1000 or more I hear no noise at all.
It’s really hard to pinpoint where the noise is coming from. Maybe the fan clutch or possibly coming from the oil pan.
I do not see any real noticeable vibration or movement on the pulleys or belt.
Can someone tell me what my RPM should be with the A/C running, car stopped in drive.
Thanks for your help
Rick