What causes popping in new wood floor when furnace turns on?
One of two things is probably happening:


The shift in air pressure is causing expansion or other movement in the wall, the floor, or both. I'd expect this to be an instantaneous effect, but it's possible it could last a while while things stabilize.
The heat from the duct below or otherwise adjacent to the flooring is heating it, causing expansion. The wood shifts where it fits under the base trim until it reaches the new temperature equilibrium. I'd expect the same to occur in reverse when the furnace stops, but maybe the movement doesn't catch and pop in the same way during contraction.


You'll want to try and narrow down the exact source of the sound. You may be able to loosen or dry-lubricate the offending point of contact to lessen or eliminate the noise.