Hi Norm,
Some motors do have a knock in them. I always thought it was gudgeons or something. Its intermittent too.It doesn't matter.
Maybe a flat /hard spot on the belt. Try another belt maybe Those automotive belts are cheap enough. Other than that I have no idea.
Sure is an interesting case. I hope you do not have anybody waiting to pick this one up. Has to be someting in the frame, base, pulleys , spindle belt or blade disc. Not much left to try is there.
All the best
Jeff
