I have had this occur and it turned out to be an un-terminated Cable TV tap. Lots of cable TV companies, if not all, use ham radio frequencies. They can do this because it is a closed system. A local cable technician said they can always tell when they have a "leaky" line because when I transmit on 2m at higher power, they start getting calls about channel 18 blanking out.
Since they are operating in a closed system, they are required to fix the problem and avoid harmful interference to licensed services. This may not be your problem, but it is a possibility to consider.
Kenneth, AB5CC