Do you hear it more at certain times of the day?
Does anyone else hear it?*bg*
Could a small mammal, i.e. mouse or rat have gotten trapped under there?
Do you have a cellar or crawl-space which you could explore for the cause?
Have you tried tapping back?:-)
Do you know Morse code...perhaps it's an SOS?:-)

I'll bet it is a critter of some sort. Unless you have a child of a certain age, I doubt if it's a poltergeist. Do you know the history of the house?

Is it possible the house might be settling and cracking its bones, so to speak? Due to excessively dry summer or other factors?

Good luck and please let us know what you find out!

kat