Jonathan Goodish wrote:
....I pulled the cover on the rear bulkhead to
look at the glideslope receiver, strobe power pack, and battery.

On your alternator whine ground loop problem, check to see how the
audio gets out of the glideslope receiver to your audio panel. My
Skylane has a rear mounted battery and the Marker Beacon receiver is
mounted on an avionics shelf near the battery.

When I was chasing my ground loops, one of the biggest ones was that
the ARC marker beacon reciever had a case ground referenced audio
output, and because of the rear mounted battery, the alternator ripple
current flowed along the marker beacon's audio shield.

The Marker Beacon has an audio output transformer, but ARC chose to tie
one end of
the secondary to the MB's case. The solution was to cut the ground
strap, and tie that end of the secondary winding to an unsed pin on the
connector. That made a true MBhi, MBlow differential floating output,
which prevents the airframe ground voltage drop between the fiirewall
and the battery from coupling into the audio...