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!
In NJ--Tierney's in Montclair, Sept. 25, 7:30 p.m.
In NYC Sat, Sept. 30, 8 p.m. at Broadway Comedy Club! Sat, Oct. 8, 6 p.m. at Greenwich Village Comedy Club.
Comedy can be a great cheap date!
Mangofy your E46!