At idle, the engine makes a strange noise that sounds almost like a bad wheel bearing, a sort of cyclic growling or muffled knocking that varies with RPM, but seems to go away when the RPMs get above around 1k (or maybe I just can't hear it over the exhaust after that). It is not very loud. It used to happen only rarely but now I can hear it much more often when the car is stopped and idling. I removed the serpentine belt to see if it was caused by any of the accessories, and the noise remained so I ruled those out. What could this be?