Really glad that you were able to tame the noise. I wanted to share two more ideas in this thread for your consideration, and for future reference. First: From the photos, it's evident that the horned balls of your dogbone shaft rotate in a parallel plane. Some folks swear that the horns should be at right angles to each other, to discourage the shaft from "ping-ponging" back and forth between the U-joint cups. It's not difficult to make a new shaft, or even to cut your existing one, sleeve it, and reassemble with epoxy at 90 degrees. Heck, some people use Toyota or BMW #0 vacuum hose to connect the motor and gearbox shafts, doing away with U-joints altogether. If the hose has a snug fit, I would bet that this is very quiet, and the elasticity of the rubber might allow the motor to get a "head start" against the load when the slack is stretched. Food for thought...
Second idea: This Weaver loco, and many of the Williams that hit the Forum with noise complaints are made of BRASS. There's a reason they don't make tubas out of die-cast metal! Brass resonates, and it will amplify any mechanical noise or vibration. Auto sound and car customizer stores sell a material called DynaXorb which is a soft rubber matting used for sound attenuation. You can buy it in one foot squares with adhesive backing. A small rectangle of adhered to the inside of the boiler does a lot to mask the noise. But as I said, you should address the root mechanical causes of the noise first, instead of just trying to cover it up.
Also, did we ever figure out what kind of motor your loco has? Good post!
