APU Noise & CPH
CPH was the first airport i ever flew to that had strict ground noise rules and what a difference it made doing the walk around you could hear birds signing.
There is simply no justification for running an APU on turn around provided stable ground power (mains ideally) is available, if your down in Spain and it super warm then conditioned ground air should be available falling that, yes run the APU for passenger/crew convenience if your airline permits, some don't !! i gather?
Ditto if it super cold, ground handlers have to work directly under the APU's and yes they have ear defenders, but they can be a safety risk in cutting off a very important sense i.e. hearing things.
The worst ramp noise i can ever recall was in LBA we were on the airbridge and at one side was a bmi Embraer, the APU was noisier than the engines and on the other side a couple of stands away was a BRAL J31/41 powering back of stand that sounded like a thousand chain saws, after that i alway picked the sector that kept me in cockpit at LBA, the ramp guys there must be deaf by now