I would have torn that wall open on day one (and possibly had the entire bedroom ripped apart by now trying to find the noise
Rather than speculating as to what could be making the noise (which seems pointless since you've eliminated all the obvioius suspects), the best course of action now is to attempt to locate the source of the noise.
I second 3eagles's suggestion that you pretend your a radar. When you year the sound, cup your hands behind your ears to funnel more sound into your ears (this also improves your ability to locate sounds) and turn your head slowly, and do this in several different places (and different elevations) in your bedroom.
Once you're pretty sure you have the sound located make a small hole in the drywall in that location (e.g., by inserting and removing a large diameter screw) and see if the noise is louder. If it is louder, rip open the stinkin' wall already and tell us what's making that noise - the suspense is killing me!!!