Cherokee Strobe Noise

In article , Jonathan
Goodish writes:

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:

I had this problem and I fixed it by adding a capacitor across the 12 volt
input at the high voltage power supply and placed a choke ahead of the input
(nearer the battery). My power supply is in the tail and the cap/choke killed
the high freq at the source.
Chuck