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Have you tried different microphones?   When you are on the ground, the
engine noise would be pretty low, and when you get in the air, the cockpit
overall noise would be high, which very well might be the source of your
noise.  Being an air traffic controller for my job, and a ham radio operator
for a hobby, I hear just about everything you can imagine.  The mic would be
the first place to start.  Do you only have one radio installed?  If you
have more than one, it's possible that they are transmitting at the same
time, and since the aircraft band is AM, this causes a hetrodine, which
sounds like a squeel, or tone when you transmit.   Another possiblility is
that you have a ground loop, or a sheilded mic cable that's ground is
intermittant, or signal wire is intermittant, and is aggravated by
vibration.   You might get on a not used frequency in your area, and while
transmitting, wiggle your mic cables, and even your mic jacks, to see if you
can make any sort of change in the sound, or hear a crackling noise which
would indicate a short, or faulty wire.   There could be alot more to it,
but this would be the things you need to eliminate first.

Ron Galbraith
LNCE -N5ES

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