I suddenly started hearing a flopping sound that sounds like a flat tire. on 2001 Lexus IS300

I noticed it going one way and not on the way back or when I went out again. When I went to the same place a few hours later, heard it again. No flat tires noted, nothing hanging. Had the bearings fixed a few months ago. What could it be? Alignment was just done. I can't see anything wrong and the car seems to drive ok.

Asked by for the 2001 Lexus IS300
Same place on same road?