Noise there and gone
 

2014 CR-V with automatic transmission, 24K miles, commuter car. Left it at home (outside) for 7 days. I get home and drive it 3 miles. Noticeable "womp, womp" from rear driver side. Next day take it to repair place. Can hear sound all the way to shop in pouring rain. They inspect rear driver tire (no apparent problem), remove both rear tires, balance, and swap them (no $$ charge). Tech says that he can still hear the sound in same general location, as can I, and recommends bringing it back for a more thorough inspection. Claims it is safe to drive. I drive it home, do some more driving, and over the course of 24 hours the sound has stopped. I can't figure this one out. Have appointment with dealer for Thursday. Looking for ideas on what might be wrong? Thanks.