I’m betting that something that’s in a comfortable silent position forward slips into a grinding position backward. A shaft missing a bushing perhaps, so it slides to the opposite extreme and there something hits something else. Not likely, but inside the motor if you’re unlucky; more likely visible and accessible elsewhere in the drive chain. Zeroing in on the location could turn out to be a real labor of love, unfortunately. If all else fails, be prepared to take the whole drive of this beloved loco apart and test each component, listening closely. If in a given component you can turn the parts both ways without any grating or rubbing noise it’s OK. If after you’ve tested each part this way and still drawn a blank, then it’s one of the connections between the parts.
I’ve fixed a lot of locos in my time, but I’m not familiar enough with this one to be specific, I’m afraid. I can say that sometimes you need a delicate touch and finely tuned ear to catch these things. On the other hand, you may quickly discover a sliding shaft or a gear that is sliding on its shaft. Another thing to watch out for, especially in a loco that has a lot of run time, is an oval hole or bushing that allows a shaft to shift sideways, or up and down, depending on the direction it’s turning. This is quite common in a worn mech. Less common, but possible, is the same kind of wear for a wheel axle bearing. I’d be on the lookout for all these things as I dismantled the loco.
I take it this is a steady noise, not an intermittent one or one that coincides with the turning of the wheels? In that case I’d be looking for wobble, but if that were an issue by now the wheel would likely be off its axle…
But if the noise has any rhythm to it at all, that’d be a clue of where to look for something eccentric somewhere in the drive.
