When I turn on the AC it starts up with a "clunk" and the same happens when I switch it off "clunk", quite loud too plus it does this clunking noise on a regular basis when the engine is idling!

Startup clunk seems to be coming from the front around belts but the turnoff clunk seems to be coming from above the heater valve!

Quite annoying and sounds like something is about to break, but what?

Any pointers?