Which axle/wheel appears to be out of round?
Often times the blind driver/axle that is actually driven by the motor will have a bit of orbital motion if there's a little extra play in the bushings.
This gives the impression of an out of round driver.
It's usually not very noticeable as you described, but one you see it ,you always will.
I have a few locos that do this. It's a non issue.
