Often as not it's wear in the vertical shaft that makes the Westrex mech noisy, the top and bottom bearings go and also the sliding gear and it's keyway are standard culprits. To check the keyway without stripping the mech down, grab firmly the sliding gear, and with your other hand try to rock the mech back and forward, you'll be able to see if there's any movement between the gear and the shaft. If there is then you're looking at a gear and shaft, as the keyway in the shaft itself will probably be worn. I don't know how prices are over there, but here it's false economy to replace bits, so we normally fit an exchange vertical shaft assembly.

One other thing worth trying. When it's chattering, try turning the shutter phasing knob a couple of turns. If the noise changes that would be indicative of worn teeth on the sliding gear. By turning the phase knob you are bringing areas of the teeth into use that probably have never done any work. Often you can quieten a chattery Westar by turning the phase knob, then re-timiing the shutter to get rid of the ghost you've just created.