You might also try placing a ferrite ring on the mike lead near the
headset. Presently the mic or mic leads are picking up the strong RF
signal from your transmitter & some of the active elements in the mike
are demodulating it. You can get ferrite chokes from misc computer
hardware, misc cables, or maybe even Radio Shack. You will have to
fiddle with the placement etc until you can get rid of the squeal.

Jim Weir (RST Electronics) would have the best answers though, and I'm
sure can fixed this problem.

Jim - are you out there?