REFLECTOR:Headset Heartache

[Brian Michalk]

> So, this morning, I got the av shop to commit a tech and I agreed
> to go down
> to the aerodome so I could run the engine for him to aid in the
> troubleshooting, since it is quiet running on batteries.  I fired it

Ahh.  There is a clue.
It is quiet running on batteries, engine not running.
It is noisy running on batteries, engine running.  -- alternator field off
It is noisy running on alternator, engine running. -- alternator field on

Perhaps it is the alternator, or something associated with a running engine.

Check/clean the ground strap to the alternator.
Check/clean the ground strap to the engine.
Check/clean the battery ground path (bus).
--those are easy.
Remove all electrical cables from the alternator, then run the engine.  See
if the problem still creeps up.

Unfortunately this seems intermittent, which makes things difficult to
diagnose.
Does it get better/worse with humidity, engine temperature or altitude?