willing to gamble that the original poster has seen sports flooring laid. batons at 300mm centers etc.

There are thousands of old homes built around the 30's-40's here, with the old wood flooring nailed directly to the floor joist. No plywood, no 45º dimensional wood subfloor. The end joints are staggered and may or may not fall on a joist. It is also 33/32 flooring and not 3/4.

Creaking and popping of wood fastened to more wood, when the heater runs, drying the wood out, is common. It is wood doing what wood does. Shrink and swell. A little earthquake in the wood to wood surface from movement.

It usually happens more, with rapid changes in conditions.