I've listened to the noise75.mp3 file and my guess would be a computer
is the cause. Possibly the power supply or video circuit. The reason for
the occasional change in tone is the change in video (on screen) and
this is causing the RFI to change its audio characteristic. This would
also cause a change in the demand in the power supply which could also
cause a change in the RFI sound.
I would bet that the sound in the interference sometimes doesn't change
for long periods of time and at other times changes rapidly. This is due
to periods of heavy use and at other times sitting idle.
This type of source will also cause TVI using over the signals. The TVI
pattern would change with the change in audio.
Within the next week my web site will have instructions on locating the
home, town home... that contains such sources.
Michael C Martin
6469 Old Solomons Island Rd
Tracy's Landing MD 20779
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