wrote:

Admin is correct as far as he goes.
This problem is also sometimes referred to as the "rusty bolt
problem". If you have a rusty bolt or rusty connection between two
metal parts on the antenna, the aircraft airframe , the transmitting
antenna and anywhere the field is strong enough the corrosion can act
as a diode and generate intermod products which is what you are most
likely hearing.

I would disconnect "ALL" of the antennas on the aircraft and use the
handheld while flying over this location. If this fixes the problem
connect one antenna at a time until the problem comes back. If the
signal is still present with all of the antennas disconnected it is
something on the ground or airframe corrosion. The next step is try
a different airplane with the same hand held. If it is still present
the problem is in or near the antenna farm on the ground.

It may be an anathema in aviation circles to suggest this, but why not
just DRIVE to somewhere near the site with a portable and see if you
get the same effect? By doing this, you've eliminated EVERYTHING
other than the site itself. If the interference is still there on the
same frequencies, you know there's really nothing you can do about it
(at least without legal intervention and/or high explosives).

Or I suppose you could fly over it in a J3 sans electrical system and
get the same results...

Mark Hickey