I’ve got an old (1980’s vintage) 0-6-0 Mantua switcher that runs great but is REALLY noisy! I’ve tried shims to raise the worm gear, but it still sounds like a coffee grinder. Are after-market re-gearing sets available?
Have you checked to see if it’s the motor, rather than the gears? An off-balance motor can cause as much noise as bad gears. If it’s a bad motor, yardbirdtrains makes full remotor kits for Mantua steam engines.
If it is the gears, try tightening the mesh instead of loosening it. Gear mesh should only be loosened if it’s binding.
Old Mantua locos (including the Big Six 0-6-0) are a bit tricky to quiet down. You can start by putting thrust washers between the front of the armature and the motor frame to eliminate the back-and-forth slop. (You should have a gear puller for this to remove the worm gear from the shaft.) You can also use some shim brass (.002" or somewhere close) in the axle bearings for the geared driver to eliminate excess slopping around. Then get the mesh adjusted with shims front or rear, as needed) to where there is a very, VERY small amount of movement when you hold the armature still and move the wheel back and forth when the mechanism is upside down. Some shims may also be needed between the axle and the cover plate. Then lube the gears and axle and try it. Sometimes you need to just run it for a while until it quiets down… Good luck.