I have 2 Proto 2000 GP 30s that have a very noticable "thump" to them. The noise comes from the trucks, seems to be on one truck each. I can't figure out what it could be. I've had both apart and all I found wrong was both locos were drenched in oil. I cleaned both up and still had the problem. I even checked each gear under a magnifing glass. To me, it sounds like there is a piece of loose balast on one of the wheels. Has anyone else had this issue with a Proto 2000? I have 3 other P2K's, and they are fine, 1 gp30 and 2 GP 18's.