My 681 clanks after every up-down motion of the smoke lever, it’s almost like a ringing but I can’t get rid of it - tried everything???
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Lionel recommend the addition of a 2026-44 spring sitting above the piston within the chamber. I have done this in many stock units and been satisfied. Also check that the 671-188 lever is in proper form/shape and that the 671-189 smoke lever retainer is fastened securely.
I've used the spring with good results, if that doesn't do it also check to see if the piston is binding
Piston is not binding and I always use the spring in my smoke units... I'm going to take a closer look at the swing distance of the smoke lever, perhaps it's bent and traveling too far ???
If the lever is malformed, it could be leaving space at the bottom of it's travel and banging into the piston on the upstroke. Noisy mechanical smoke units are not uncommon.
Mine has always had that "ring" and clank to it.
I thought it might be a characteristic of them, I’ve heard others do it also
https://www.youtube.com/watch?v=QpZQI8q6JuY
Start at 4:25 and see if yours sounds like that. I guess the sound I hear in that video is normal.
John
I have to chuckle looking at a turbine locomotive with chuffing smoke. 
Yup, that’s the sound….
