Michael 182 wrote:

All right, I did some Google searching (sorry - I should have started there)
and now suspect that my problem is in the grounding. But where in the
system? Can I fix this, or is it back to the avionics shop?

One thing you can do yourself is to isolate the jacks. Buy special fiber washers
that have a double-thickness cylinder in the center. Remove each jack from the
panel, re-drill or ream the hole in the panel to fit the center section of the
isolation washer, and re-install the jack with a plain fiber washer in the front.
The two fiber washers keep any metal parts of the jack from touching the airframe,
so the only way the jack is grounded is the ground wire to the intercom.

George Patterson
Love, n.: A form of temporary insanity afflicting the young. It is curable
either by marriage or by removal of the afflicted from the circumstances
under which he incurred the condition. It is sometimes fatal, but more
often to the physician than to the patient.