[Rail-Rider]

DoubleOO,
It would be helpful to know what make and model this is, it's transmission type and possibly the locos's age.

The suggestion as to running it off the track is a good one as that should point us towards a motor/transmission problem or a wheel/flange/traction tyre problem.

 

[Rail-Rider]

I think if it was meant to make that noise, it would make it running off the track as well as on it.

If it runs smoothly and quietly off track, then it seems the problem has to be with axles or wheels/flanges and might have something to do with the track itself.