I had a DVR (digital video recorder) that was constantly recording the video feed into hard drive. It was in my bedroom. One day when I was cleaning I moved to box and didn't think much of it. Then in the evening I was getting ready to sleep and on the side of the bed where I sleep I could hear extremely loud boomy sound. It was driving me crazy, but as soon as I moved to find the source the sound "disappeared". Going back to bed and it's back again. I was going around the room like crazy trying to find the source, or maybe it was outside? Eventually I put my ear to DVR and noticed the sound was quite similar, but very faint. By doing this I moved the DVR and when I returned to bed the sound was gone. This was so frustrating. But I least I learned something how to find a source when something like this happens next time. Basically the unlikely positioning of DVR, my furniture and myself created loud resonance.