My floor joists's make a crack sound.
I'm a newbie, too, and not an expert in this area, but I see you've been waiting for an answer since Halloween.
I've often noticed sounds made by materials expanding and contracting due to heat/cold. I believe it is more common to get these noises from metal than wood, but they can come from almost any material. It's possible to get these sounds from just a single piece of material, but I think that case is rare. More common is two pieces of material joined together, which have different rates of expansion/contraction -- one material expands more rapidly than the other, and the result is the creaking and popping sound when the force overcomes the friction of the joint...it's a little like how an earthquake fault stays dormant for years, building up the pressure, and then lets go in a big quake. I think you're hearing the "quakes", as the pressure is released.
If you can open up the area a bit, and find the place where the materials are rubbing, you may be able to force a thin metal or wooden shim in between them, letting the movement happen without the buildup of pressure, so no noise (or at least, a lot less noise). Even driving a nail between them may help.
Again, I am no expert, so my suggestion may be just a load of BS. Anybody else want to take a shot at this one?