I went for a ride in some nasty conditions a few days ago and now I have a small like thunk in the drivetrain. it happens when you're pedaling slowly and under pressure, like on a climb. ive cleaned all the **** off the drive drain and attempted to locate it but I can't recreate the conditions when I'm not on it. I put a new chain on and tried that but still felt it. The crank is a Shimano M521. anyone ever had this problem or know where to locate it?