strange com fault
 

I have a king com on a piston heli
on the ground it's ok but when in the air it gives background noise and if I alter the throttle on the piston engine it sounds as if I can hear the engine accelerate and buzzes faster
If someone transmits I can hear them perfect and I can transmit perfect
Any one got any ideas

Sound to me like you've got either alternator or ignition break through.

Have a chat with your avionics engineer to see if you can improve the suppression on the power supply to the radio/intercom!

The dog.

Should be a capacitor on or near the alternator. This could be U/S. The buzz will increase with RPM. If you were in a fix wing at buzzing would cut out at about 2200rpm as it exceeds your limits of hearing.

Get a avionics guy to test that capacitor. Easy fix.

If its an R22 try removing the dual cyclic, the internal connection has been known to give problems.