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!
dmax on the radio--answering all your car questions as best I can!
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