My goodman 5 ton heatpump / a/c compressor when reaches desired temp makes a heavy (loud) buzzing until I lower stat To demand more cooling or turn it off. Once it is off for 30 secs I can turn on it is fine. The noise is intermittent it doesn't do it every cycle.!

Where is the loud buzzing coming from?

It almost sounds like a contactor chattering. When it does that push down heavily with in insulated object on the contacts in the contactor to the compressor.

The noise is coming from the heatpump/a/c unit outside it is more like as if at the point of the noise the stat is satisfied but not shutting down.

Yea, relay chatter. Could be stat. Could be contactor. Could be loose wiring.

Thanks, just out of curiosity could it also be the reversing valve not holding for some reason?