headset feedback loop during xmit

My Telex 30 headset and SL30 nav/com have developed a loud and annoying
oscillation during xmit. The tone is both obvious in the headset side-tone
and also is heard by other's listening on the frequency.

I've tried two other headsets (a DRE ENR and a non-ANR headset) and both
work fine with the SL30. My com2 is a King 97a and it works fine with all
of my headsets, including the Telex.

The problem seems to have developed in stages over the past several months.
First I was noticing the Telex and SL30 would occasionally give my voice a
hollow sound for the first few transmissions of the day, then sound fine.
Finally last week the tower controller mentioned the squeal which I was also
noticing. It tends to go away after a bit but it sounded odd for the entire
flight.

Has anyone experienced a similar problem? I'm hoping to try a few other
brands of headsets this weekend to see if the headset is the likely culprit.

Best Regards,
Mike.

http://flickr.com/photos/mikenoel/