Headset Noise

I have a Sigtronics SPA-400 intercom in my Cherokee. Everything works
fine, except that the right seat position has an annoying amount of
electrical noise in the headset. It is a high-pitch whine (alternator
noise), and the pitch seems to follow RPM. This is not noticable in the
pilot's headset, even with the headset volume at full. It is much less
noticable in the right seat headset with the headset volume turned all
the way down, but obviously this makes things harder to hear for the
right seat occupant (often another pilot).

In an attempt to rectify the problem, I replaced the intercom wiring
with shielded Tefzel cable, replaced all of the jacks, and isolated all
of the jacks from the panel with fiber shoulder washers and brought
everything back to a single point ground. That helped, but only slightly.

I'm trying to figure out what's causing it or, at least, how to stop it
on that particular jack. Obviously, the intercom wiring runs adjacent to
the electrical buses on that side of the panel, so I'm not sure what
role the buses may play, especially since I'm using shielded cable.

Does anyone have any ideas?


Thanks,
JKG