This particular locomotive, a 2016, I had not used in about a year. It started fine and ran for a few minutes and then I heard what I would describe as a grinding noise. The noise was not constant. It came on and off with maybe 1 second between the grinding noises. Like it came on with the rotation of something else. I removed the engine from the shell. All wheels moved very smoothly by hand without effort and I could see or feel no binding whatsoever. But if I put down the engine on my test track I could hear the grinding sound. (I should have said that if I ignore the grinding sound the locomotive will eventually stop.) I greased the gears, but that didn't help. I'm a bit surprised I can't see or feel the problem by turning the wheels myself. Any ideas?