Eep!
Well, I'm not an expert, so consider this only an objective opinion. However, when I had similar issues with my Westinghouse, it was because the end bells of the motor were dry, and it had caused the sleeve bearing in the front of the motor to fail. Fortunately, it was a common bearing, and Roger was able to get a sleeve bearing at a local shop for it.
If that's not what it is, then I'd wonder if the impeller itself has become unbalanced somehow. Did you have the motor apart at any time, and/or oil the felt at the ends of the motor?
Like I said, I could be completely off-target, but that would be my first thought...
--Nate