John - have you kept a log to note if there is a pattern to days / times etc that the interference is there? Is it just there when the highway guys are there, for instance? Gawd there could be so many things that are causing it. In the years when I was living off grid power, my 12V CFL fixtures produced a noise similar to what you're describing. They actually have an oscillator inside them that coverts the DC to a high frequency AC that the bulb can use. Sine and square wave inverters can do that also. The highway garage might possibly have something like that in use. Also, the highway guys probably have several handheld radios which plug into a charger every night. The charger could be a source of RFI. Any solar panels close by? The PWM charge controllers throw out too much hash to be your culprit but older reflexive units can produce oscillations, depending on the particular circuit. As Dood said, you might try walking around with a portable to have your own "bunny hunt" at your QTH. Good luck!
