I have a neighbor who has a plasma TV with a switching power supply. It's noisy all the way up to 440 with a chugging noise that happens to fall on the input to my repeater. In my case, it's not on all day and when it is, it only takes out weaker signals. I put a voting receiver up about 4 miles away, so my problem was somehow mitigated. It didn't make the interference go away, I just found another way to deal with it.
Sad to say, I don't know my neighbors (my kids do, and it seems that not knowing some of them has saved me much drama...), so I chose the path of greater resistance. I also don't want to be the guy to suggest to someone they can't watch their thousand dollar television set when I might want to get a permit to put up my tower later this year. These days, people come and go in the neighborhood and some are... gasp... non-hams or SWLs. In fact, we call the new guy next door the Lorax, because he cut his trees down and had the nerve to ask me if I liked my trees (they apparently shade his in-ground swimming pool for part of the day... I don't have a pool).
So, how are your people skills? You'll have to diplomatically convince your neighbors to let you in and help you find it - and when you do find it, to let you modify it to reduce the noise (how are your technical skills, too?). That AM radio will probably help you, as would a scanner with an AM setting or handheld aircraft band receiver.
