I'm looking for advice to help shorten the trouble-shooting process here.

My Arrow has developed an alternator whine over the past few hours. It
is *barely, barely" noticable on both radios when receiving, but loud
and obnoxious (both to me and other aircraft) when transmitting. No
doubt it is the alternator - switching the alternator off fixes the
problem.

There do seem to be some variations - the noise is worse when
transmitting on the pilot side than the copilot side. Worse when using
the installed PTT and headset jacks than the mic.

I've got an an old KMA-20 and a Sigtronics SPA400 intercom installed.

Nothing has 'changed' recently. The alternator was replaced by an OH
about 18 months/100 hours ago. The battery was replaced 4 months/25
hours ago at annual. Things were fine for the first 20 hours after
annual.

Because it was quick and easy, I cleaned the battery connections and
the pins on the back of the KMA-20. No improvement.

I'm time-constrained, and so would rather not have to make multiple
trips to my AP and/or radio shop. Any informed guesses are more than
appreciated.

Thanks.