Tauno Voipio wrote:

An educated guess:

Short from microphone connector ground to chassis.
The microphone should get its ground connection
from the radio (or audio panel) only, to prevent
the stray ground currents on chassis from creeping
onto the audio line.


Bingo,

Proper installation of aircraft mic and headphone jacks requires the use
of insulating bushing where the threaded sleeve on the jack would
otherwise come in electrical contact with the airframe. The wiring
behind the panel from each jack requires that there be three conductors
(one each for tip=ptt, ring=micHi, sleeve=micLo & Ptt return) surrounded
by a shield. Good practice connects the shield to airfame ground at the
radio end.

None of the mic or headphone jacks should be touching airframe ground.
None should utilize the airframe as the return conductor for any audio
signal, mic or headphone!

The sleeve connection from all jacks should go to a "single-point audio
grounding point" at the rear of the audio panel tray.

The factory installed "original mic jack" usually did not follow the
proceedure cited above. It is likely that the alternator whine is
getting into the transmitted audio due to a ground drop between where
the radio is grounded and where the mic jack touches the airframe.