Hey, I had some 80's white oak (2.5") floors expanded into the kitchen and then all of it refinished, 2 weeks ago. Occasionally in the middle of the night I wake up to loud popping sounds from the floors. It seems to do it when the weather is particularly cold outside at night after being warm during the day. Keep in mind its mild right now, 71 during the day and 62-63 at night inside.

The original floors never made a sound before, and seem to make noises now - it is not just the new wood that seems to be doing it, the original stuff is the noisiest. Is this just everything settling in and sliding as things contract? Can I expect it to stop in time? Perhaps this is normal behavior.

I noticed that even in the old installation places, there is 0 expansion gap between the wood and the wall in quite a few spots. Never had buckling or any problems before, so not sure I can attribute it to that.

Should I cut an expansion gap where tight with a flush cut saw before I put baseboards back on?

The finish may still be curing. It takes almost a month in some conditions for full cure. The older flooring may have not been fastened down as well as the newer floor. If the older floor is cut net to the walls and was not in bad shape before sanding recently, then the new floor may be good, too. You will have to wait and see if more serious problems will arrive when the floor finish has cured.