This is for all you avionics buffs.  I was headed out for an instrument proficiency check last Saturday.  As we were taxiing out I noticed what sounded like a slight echo in my headset.  My CFII heard it too.  It was the kind of thing one hears from a feedback loop on public address microphone.  We didn’t give it much thought at first but as we continued talking it got progressively worse until finally it became a squeal whenever we spoke, again, just like a PA system microphone.  We played with the squelch, volume, and different headsets, all to no avail.  I later found that it isn’t there with only the pilot headset plugged in.  It also disappears when I bypass the audio panel.  This happened once before several months ago.  I had it look at by a reputable avionics shop but they were unable to replicate it and could find nothing wrong with neither my radios nor my audio panel.  I plan to have them take another look as soon as they can schedule me in but thought I’d query the list in the event anybody here has any experience in this area.  Your comments, thoughts, and consideration are appreciated.

Danny
LNC2-360 Mk-II
AOPA 04856278
EAA 647062
NRA 156453324
Nothing is foolproof to the sufficiently talented fool.