The idea is that just the very end of the path needs a precisely sized hole. Every other part of the path is wider, what reduces wearing, and doesn’t need as much precision, so wearing isn’t a problem.

I have no idea how it actually works on practice. I never used one of them, and I never used an abrasive material.

Do you think those really tiny bearings used for the extruder axis’ can make such a sound?

Absolutely, yes. If it never made that sound and started with age, do not go sanding anything. Clean and lubricate again and look for broken parts.

Any point of contact can be making that noise. Including the motor, yes, but everything else is more likely.