Thanks to all for the suggestions on this topic. I spoke with a guy at
Electrosystems (now Kelly Aerospace) about this particular problem and
he mentioned the most common cause as one of the following:

1 -- One or more failing diodes;
2 -- Inadequate grounding between engine and airframe;
3 -- Inadequate grounding between avionics and airframe;
4 -- Deteriorating alternator circuit breaker.

He provided instructions for testing each one of those possibilities
using a VOM. Specifically for #1, he instructed to measure AC volts
output on the output post at moderate engine speed (1700-1800 RPM) and
moderate load. If the AC volt reading is greater than 1 volt in this
configuration, the (Chrysler) alternator is out of overhaul spec and the
rectifier bank is probably starting to fail. If the AC volt reading is
under 1 volt, the diodes are fine. My alternator has measured around
0.55-0.60 volts in the configuration mentioned above, so I assume that
means that the alternator is fine, and I most likely have a ground issue.

In my Google search of previous threads, someone mentioned that a
panel-mount intercom installation with locally grounded jacks is a very
bad thing when it comes to noise. My Signtronics SPA-400 intercom has
isolated (and single-point ground) for the mic jacks, but the headphone
jacks are locally grounded. Sigtronics insists that this should be fine
and not contribute to noise assuming that there isn't a ground problem
elsewhere.

Any input on the above?


Thanks,
JKG