When playing concertina with my hearing aids in I get a marked tremolo effect going on. I am guessing that the hearingaid manages to shift the pitch by a couple of hertz, and that and the original unboosted signal are recombining in my ear to give the tremelo effect.
Not a great problem, I simply take the hearing aids out, but I'm curious if others have come across this?
The hearing aids are NHS standard behind the ear things, with 'open backed' ear fittings.