Lightspeed PFX background noise transmit

I bought a used Lightspeed PFX. Nice unit but when transmitting at high power setting (IO-360 RV-8) I get lots of background noise in the transmission. I've tried different mic muffs, position of mike, etc. and it hasn't changed. Any tricks or does it need to go back to Lightspeed?

Did you try a different headset to rule out a problem with the avionics?

Yes, other headsets do not have this problem.

shot in the dark - if you have Garmin radios, look at the sidetone levels. Have seen examples where settings for headset 'A' make hash&slash with headset 'B'. Ditto with the squelch settings.