We had a similar problem here years ago, caused by a city fire dispatch transmitter 30 miles away. It had a spur that we could follow around in frequency too. Our repeater is on 2M and their transmitter was on 154.37, if I remember. Their transmitter was the entire cause of the problem; no mixing was involved.
I'd carefully check that, in fact, the suspect transmitters were disabled for the tests you mentioned. The linking scheme to them is also suspect as suggested. Are you hearing any other audio in the spur? If not I think it's a dirty transmitter; no mixing or intermod involved. Laryn K8TVZ
