Cherokee Strobe Noise

I know that this has been beaten to death, and I have searched the
archives, but I'm trying to narrow my specific case down a little bit.

I have a Cherokee with factory-installed wing tip strobes. I have an
original (1977) King Silver Crown avionics package, including audio
panel, KX170Bs, etc. I also have a Sigtronics panel-mount 4 place
intercom that was installed at some point before I purchased the
plane... I believe it is a SPA-400.

Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets. The siren sound is also occasionally transmitted
over the radio when we make transmissions, because I have been told to
"check my ELT" after making transmissions. Here are my observations:

1 -- Noise is present with intercom on or off.

2 -- Noise is WORSE when any radio, but especially marker beacon, is
selected on the audio panel.

3 -- When all radios are unselected on the audio panel (i.e. nothing
selected to either speaker or headsets) noise goes from very noticeable
to barely noticeable.

4 -- Noise is much WORSE when things are "cold." After about 20 minutes
of flying time, the noise is still noticeable, but not nearly as bad as
it was when the flight began.

5 -- Unless I'm hearing things, which is possible, I can SOMETIMES hear
the noise through the headset with the strobes on but avionics bus OFF.

I am not sure whether the strobe power pack is crying out for help,
whether I have a ground problem elsewhere in the system, or whether I
have an improper installation of something (like the intercom), or
whether the audio panel is shot.

The strobes were factory-installed, so I assume that the installation of
the strobes is correct. The SPA-400 intercom was obviously not factory
installed, and I am not convinced that insulating washers were used on
the headset plugs, so I do need to check that out. I do know that
shielded cable was NOT used on the intercom installation.

Does anyone have any ideas? I've basically narrowed it down to three
things, in no particular order:

1 -- intercom
2 -- audio panel
3 -- strobe power pack


Thanks,
JKG