I've had 2 Altronics headsets giving the exact same problem. Its in the head set them selves. The problem is not there with other headsets (tried david Clark, Bose, Lightspeed, and a couple of others) Have tried ferrites, caps, changing lead lenghts, earthing, isolation transformers, isolating and changing earths (all in conjunction with a RF test set). They all changed the amount of RF feed back thru the Altronics headset, some better, some worse, but none never fully fixed the high pitched tone on TX. It was definitely RF induced feedback (as tested on the testset and using various RF power levels and loads). The Altronic headsets are now like Geoff's, sitting under the bench in the hanger.
James