Mysterious "knock" coming from an interior wall

[Jonna Weber]

I've had an issue in a condo I manage for a couple of months and after much trial and error, I thought I'd check with BP to see if anyone might have an idea what might be going on.  The condo is a newer unit (2004) on a first floor.  My tenant has been complaining of an occasional knocking sound coming from the upper half of an interior wall.  We have run all the water in the unit and in the unit above it continuously, flushed toilets, etc. and have not found a correlation as to when the knocking comes.  It also does not seem to be tied to when the furnace powers on and off.  It happens more during the middle of the night than daytime, but daytime knocks happen too, and are getting more frequent.  It is a single loud "knock" vs. a tapping.  My plumber says that he would have to open up a wall to see if he can figure it out.  I really am not looking forward to going that route, as I'm thinking there is a good chance he would not find anything.   Thanks for reading!