I hear pops and crackling sounds, like walking on top of rice crispies when I walk on the hardwood floors.

I have also heard loud popping sounds from the hardwood or plywood (not sure) on the 2nd floor from the first floor. They don't occur often, but I do hear them form time to time(1/day or 1/every 2-3 days, etc). What could be the cause of this ?

thanks, for all of your input. It is greatly appreciated.

I might try to install 20 sq ft this upcoming weekend on a separate room, and see(actually, hear) the results. I'll post the results afterwards.

To one of the questions above. I do not have any underlayment. It's just 3/4" plywood on top of the joists.