A well hidden rock or nail/screw in your tire. You at least might want to doublecheck the air in the tires while you've got a minute or two.
I don't think you need to start thinking about serious mechanical things yet...many things thump...sort of...but what you describe sounds mostly like a rubber on something...and before you start thinking of broken parts...sounds like you need to spend more time looking with loyal friend!
Performing at the Comedy Cove--595 Morris Ave. Springfield NJ reservations 973 376-3840
A recent set