Have you tried turning the active switch back on again, after turning it off? I've seen circuits (especially feedback circuits dealing with noise) that seem to work fine, but over a long period of time can subtly build up an oscillation (e.g. a squeal) that eventually swamps the system - but resetting them puts it back to square one.
Pilot Communications put one of the headsets on test for 8 hours without the fault developing again, which is why we thought there could be an incompatability/interference issue somewhere in my flying environment.
I'm only a student and tend to have my hands full most of the time
so "stop the noise (distraction)" and get on with what I'm doing has always been the priority.
I have to admit to not having tried a simple on/off yet. I'll ask my instructor to humour me next time I'm up, and fly the airplane whilst I "twidddle".
Should a straight on & off do the trick or should I try off, wait a minute, and back on ??