Jonathan Goodish wrote:

That's very helpful, but unless I'm missing something, I'm not sure that
it explains why I'm only having the whine problem on the right side of
the cabin (in both front and rear jacks). All jacks are isolated from
the airframe, and I'm running a common ground back to the intercom
ground. The braided shield is floating at the jack ends and grounded at
the intercom ground point. The whine on one side of the cabin would
make more sense to me if the jacks were grounded locally to the
airframe, but not sure how noise would enter (or exit) the system on one
side of the cabin with the intercom system on a single-point ground.

I concur. I dont think it is a ground loop issue. I'm thinking a wiring
error, or short which occurs only when headphone plugs are pushed in,
or an internal fault inside the SPA400. Too bad you dont live close, I
would lend you mine to see if it made a difference.

Outside of the stuff I've posted to the newsgroup, one other oddity:
when I activate the marker beacon output on the audio panel, the whine
is more noticeable.

Obviously, it sounds like there's a ground problem somewhere, but I
don't know where to start. I'm baffled as to why I'm only hearing the
problem on one side of the intercom system and not the other, with none
of the jacks or PTT switches grounded directly to the airframe. The
noise must be alternator-induced, because it is only present when the
alternator field is activated. If something was looping through the
audio panel or somewhere else, why would it only affect the jacks on one
side of the aircraft?

The SPA400 drives the pilot headphone jack differently than the other
three jacks. Is the problem only on the copilot's jack, or also on the
back-seat jacks?

I'm puzzled by the "crackle". The alternator whine caused by a ground
loop is usually musical, pitch varies with engine speed. Crackles are
usually vibration induced intermittent contacts such as an improperly
seated audio panel, or nav-com. These usually behave with the engine
off, but can be induced by bashing the panel with your hand. Crackle
could also mean loose electrical connections to the bus, unshielded
P-leads, noisy electric-powered gyro instruments, bad fuel senders,
vibrating point mechanical alternator regulator (hopefully yours is
solid-state) or a vibration-sensitive Over Voltage Protection Relay. If
you have a good DVM, connect it to main bus, and read the voltage prior
to engine start, during runup and flight to see if there are any
fluctuations which correlate with the crackle you are hearing...