This is my guess of file name: n6kj-rfi.mp3, submitted from Kelly, N6KJ in your unidentified RFI section.

I feel that the operator hit it on the head. I would suspect a switch mode battery charger with a fully charged battery attached to it. You can hear it kick in and the frequency rise. This is the pulse charge function, the tone rises as the charger delivers less charging current. If it is fairly constant as they describe then it's time to investigate if the neighbor has a boat or motorcycle in storage that they may just have a maintaining charger attached to. Pulse chargers are one the best ways to preserve lead-acid battery life during storage. If the noise is seasonal it could be some sort of personal rec vehicle on a charger. Another thing to investigate is watching the noise versus outside temperature. The battery will pull more charge current when it is colder, if it starts at a lower frequency (audio) or lasts longer when the temp falls then you can conclude it's outdoors.

I had a similar problem with the maintenance charger that I had attached to my garden tractor during the winter months. The solution was as simple as mounting the charger under the hood of the tractor, this effectively shielded it and the noise went away. Instead of a 12V Molex going to the tractor I just had an AC connector to attach. This proved even more flexible due to the fact I just had to have a 120VAC source to store it instead of remembering the charger if I had to move it's storage location (it went to a friend's house for a while). This is a good way to convince the neighbor to try re-locating it :)

This is the first time I have seen your site and it's very interesting. I will continue to check back often because I love finding strange noises. I worked in pro audio touring dealing with tons of wireless then switched to broadcast engineering. I was always included on the noise hunts! Now that I have switched employers I don't get to do much "sleuthing!"

Chad Wahls

Electronics Engineering and Support
University Of Illinois
School Of Music
Urbana-Champaign