Sydney,

What you have here is a pretty classic case of intermodulation interference.
Judging from he results of your investigation (good work on that, by the
way) I would say that the most likely cause is the frequency mixing
("heterodyning") of two very strong out-of-band signals in the "front end"
of whatever radio you are listening to. Yes, it is possible that the mixing
could be taking place in some corrosion of an antenna mounting or some such,
but that is, in my opinion, far less likely.

It's a good bet that at least one, and very likely both, of the very strong
out-of-band signals is in the commercial FM broadcast band. There may be
several such signals being broadcast at high power from the antenna farm.

At this point the question is whether the interference poses a threat to
safety or merely an annoyance. When the interference "breaks squelch while
you are tuned to the TRACON freq are you still able to hear the controller
when he/she transmits?

It may be a good idea for you to notify the FAA about the problem. Further
investigation may involve the use of a spectrum analyzer, not a tool you are
likely to have laying around in the garage. If the Feds determine that the
potential for intermodulation interference is a hazard they can take
corrective measures, such as reassigning the TRACON frequency in the area to
one that has a lower risk of interference from whatever signals are being
transmitted from the antenna farm.

--
-Elliott Drucker