I've been working in the woods since '75 and have heard only 2 sounds that might be what you describe:
1. Tree trunks hitting each other in the wind, a low pitched bonking sound. If no wind, that's N/A.
2. Once I heard what sounded like a series of gunshots a hundred + feet away. Probably 4-6 in fairly quick succession. Then the tree fell, landing about 50 feet from where I had been. The popping was the roots breaking, first one, then another became overstressed under tension and it popped, then others more quickly. No wind that day, although it had been windy a few days before. Some of the roots showed evidence of rot. If that's what you are hearing, I would expect you would have had a tree go down by now.
My time was spent in coniferous forests; might be different with hardwoods.
I don't recall ever hearing any noises from expansion/contraction.
How long have you lived there? If it were limbs banging together I'm sure you would have heard it at other times and especially if there were a breeze.
I wonder if a critter was making the noise?