Inspired by the many helpful suggestions of group participants, I
finally went searching for the potential ground loop issue--and I
believe that I found it. The tip terminal of the copilot mic jack was
contacting the airframe from below, creating the loop through the
copilot circuit of the intercom. Unfortunately, this was by far the
least accessible set of jacks in the airplane (Cherokee), but I was able
to get a dental pick up there, hook the terminal, and pull it down away
from the point where it was contacting the airframe. The other
terminals have wire connections and were soldered and covered with heat
shrink, but since the tip terminal wasn't connected, it was missing the
heat-shrink.

The intercom system is now almost completely silent, though I've only
ground-run the airplane to test at this point. There is still a very,
very slight whine that is audible through the system when the headset
volume is at maximum, but it is not obvious and it is now equivalent
between the pilot and copilot positions--the nasty whine that was in the
copilot headset appears to have almost disappeared.

I have no idea whether this will have an effect on the issue of static
in the sidetone when the pilot mic is keyed, but who knows. I'll try to
fly the airplane over the next couple of days to see what happens, but
hopefully this problem is now resolved.

Thanks to all who responded with input on this problem.



JKG