Have YOU tried it with different types of headset??
I have noticed that with Avcomm headsets the mic or the leads pick up the radiated transmit signal, which can cause a feedback or squeal in the sidetone, use a David Clark or similar and the transmissions are fine.
This happens when you have a SPA400 intercom with the insulated mic sockets.
At the low end of the freq band you will find it ok, the top end poor transmissions.
Make sure the antenna system for that radio is ok, and get a Avionics Eng to do a VSWR check.
Make sure that the mic sockets are ground properly.
It is one of these silly radio things!!