Switching off the intercom and it goes away.

........................

Any final suggestions?

Dom,

 

The easy one is --- Leave the intercom off.  Or, only fly from the right seat and let the passenger go crazy.

 

I take it that recipients of one of your "G'day mate" telecommunications does not hear the piggy squeal.  Sacre Bleu!

 

I take it that this is not feedback from the mike to the headset - either externally or through your teeth (connected to the jaw bone, connected to the head bone...........).

 

It must be in the audio wiring for the left side (Duh).  Ground loop (shield connected at both ends)?  Wires passing by some electronic marvel (AP, air data computer, GPS, etc)? So, does moving the left side wires around change the squeal? 

 

You are slowly drifting into the vortex of a Black Art. But, I am sure that somebody on the LML knows the secret society handshake and various electric gimcracks that might give you some relief.