Actually, it's probably what I said it was at first, solar gain. The sun coming in the windows acts as a magnifier and bakes the floor, raising the temp considerably. This causes the floor to lose moisture rapidly (because it is an unstable wood, birch, similar to maple). Now you get snap, crackle, pop. Remedy? Cover those windows and see what happens. No need to black them out, just diffuse the light with shades or blinds.