I have read older posts that attribute this thumping noise at higher speed to a cracked gear, but cannot find any diagrams that show power truck gearing or how to access it.  Further, I note that the noise only occurs when the Doodlebug is running on the rail, not when wheels not touching rail, so not sure if this is cracked gear.  I've converted to DCC sound and couldn't test on DC analog so now am hoping to find a solution as the thumping noise really detracts from the ESU gas electric sound.  Thanks in advance for any help.