I'm not into the mechanics of the machine, myself. But, you can contact Shopsmith directly and ask the question. They are excellent at answering questions!
I'm thinking bearings in the quill feed. It sounds like the play in your quill is also causing some vibration when you use the lathe.
That's my two cents. I hope it means something!
[font="Comic Sans MS"]Shannon Fields
US Army Retired![/font]
[font="Comic Sans MS"]Bryan, TX