The Pittman motors I've seen in MTH locos were not of the ball-bearing variety. (Note there are ball bearings in the gearbox which absorb thrust loads from the worm gear.) So it's entirely possible that the bearing could be dry, or even worn a little oversize and crying out for lubrication.
What's really good about MTH is that the worm gear isn't pressed on the motor. Even the flywheel comes off easily with a tiny Allen screw. So you just have to find another Pittman motor. Try to match the first four numbers: If the original loco had a 9432xxxx, you should replace it with a 9432 to ensure comparable performance. Pretty sure that a 9433 or 9434 will be too long in a Mikado.
A 9412 would also work, but the '3' in the 3rd position indicates rare earth magnets which make the motor more powerful. So a 9412 would give you similar size and RPM but less torque.
Pittmans are labeled with the nominal voltage of their windings. It would be ok to substitute a 15.1 or even a 19.1-volt winding. But I wouldn't use a motor wound for less than 12V (factory spec.)
My $.02. Hopefully a drop of oil cures it, at least for a little while.
