("Jonathan Goodish" wrote)
[snip]
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.


I know zip about electronics ...but am still (miraculously) correct 75% of
the time with this type of post.

It's "always" the ground. g


Montblack-wire-goes-where?
"Dance, mailman."