Up to 175 knots my radios and audio panel function perfectly.  At about 175 knots plus I start to get a lot of noise through the headset  as if the mic has opened and let the aircraft noise into the system.  I have tried playing with the squelch to no avail.

Ian,

 

I have the Terra audio panel and intercom and I know I have to change the intercom squelch after transitioning from the ground to the air because the increase in noise triggers the intercom.  Something about taxiing at 1100 rpm versus 2600 rpm in a climb.

 

Have you tried to just turn the intercom off?  Since I often fly alone I usually have the intercom off - thus no adjustment is necessary.

 

Is the "side tone" circuitry too sensitive?  Of course, this should only happen on transmit since some part of the audio panel or radio produces the "side tone" so you can hear yourself as you transmit.  There may be an adjustment (there is in the Garmin 430 radio part).

 

If it were static related, you would probably hear it as static or squealing during such things as receiving a radio transmission........

 

I once had trouble with a noise canceling headset when wind (wind or heavy breathing) across the mike would overpower its circuitry.  Of course I tried to use it in an open cockpit airplane.  If you have such a headset, try turning off the noise canceling feature.

 

Uh, wrap your mike in some of Dan Newland's sound killing stuff and you will probably have to yell at the tower controller - always a satisfying exercise (especially if you pretend to speak in a foreign tongue), but maybe the noise will go away.

 

Have you often heard things that aren't there?