I've had a weird problem going on for a month or two now. I'll be home watching TV or reading and all of a sudden, I hear a single loud knock from inside my living room wall. Not scratching, just a knock...as if someone punched the inside the wall with a fist or rubber mallet. Usually I'll only hear it once a night, although sometimes I've heard it two or three times in a single evening. I don't think it's an animal...I don't hear any other scratching or movement. Just before Thanksgiving, I bought a new TV and had it wall mounted. I had an electrician come in and install a new outlet about four feet up the wall. He also cut two access holes for running the HDMI and optical cables. He did comment that it was strange the wall had no insulation because it's an exterior wall. Anyhow, the sound I hear from the wall is generally a foot or two to the right of where the TV is mounted. I don't think the sound is water hammer...there is no plumbing in the wall. I've read noises can sometimes be caused by HVAC systems cycling on or off. There is a floor mounted vent, but no return along the wall. I had a ton of exterior work done two years ago...new roof, siding, gutters, etc. The area underneath the living room was once a dirt crawlspace, but I had it sealed with concrete and a sump pump installed. I've never had water down there. Other than the bogeyman or a poltergeist, any other guesses at what I'm dealing with?