Ditto the tire pressure comment.
Lately I've had grit in my bead from airing down.. Haven't un-seated and cleaned in a while.... Every so often I'll start to pull a bit... Usually means it's time to check the pressure in the passenger-side rear tire. I can usually tell the most when it gets down to about 20-ish psi with the other rear still sitting at 30. That's just enough height difference to cause the pull.