Good suggestions, Lloyd, but I think the possibility should be kept in mind that the root cause of the problem may be the power wiring or delivery, but that root cause may in turn have already damaged a component (such as by causing excess leakage in a capacitor or transformer), which in turn may be what is causing the audible symptoms.
So moving the system elsewhere may not be conclusive. The same symptoms could appear elsewhere (as they apparently did at the friend's house), suggesting a bad component, but a replacement component could subsequently suffer similar damage when exposed to the defective ac source.
In my (extensive) experience as an electrical engineer, whenever problems arose that were very difficult to isolate, and experiments gave conflicting or confusing results, often it turned out that two inter-related problems were present at the same time.
Regards,
-- Al
So moving the system elsewhere may not be conclusive. The same symptoms could appear elsewhere (as they apparently did at the friend's house), suggesting a bad component, but a replacement component could subsequently suffer similar damage when exposed to the defective ac source.
In my (extensive) experience as an electrical engineer, whenever problems arose that were very difficult to isolate, and experiments gave conflicting or confusing results, often it turned out that two inter-related problems were present at the same time.
Regards,
-- Al