Might have someone look at it or listen, hard to say.

Recording might help us. Nail in the tire?

 

does the flop sound increase or decrease with speed?

 

The sound blends into one sound that is louder.

 

Any vibration while driving?

 

Carefully inspect all your tires for a bulge or blister indicating a tread separation. That is a very dangerous condition which will almost certainly result in a thrown tread and blowout. A tread separation will damage body parts and possibly suspension parts.