My backyard backs in to a strip mall with an auto mechanic shop. We hear a high pitch sound some times of the day and when we step outside to inspect, we only hear a different sound from the shop behind - a typical machine running, but not necessarily unpleasant. Turns out the walls are able to mask the lower pitch sound somehow and let in only the unpleasant higher pitch sound to the inside of the house. I am not sure how to explain it to the shop owner, but we are just going to ignore it for now. I guess I need to look into insulation options :)