I had exactly the same problem at first. The unit was originally mounted on a timber chassis on those cedar brackets you see on the wall, which is actually the 2" thick oak boards of the original barn, now sheathed outside in California Redwood, but that's another story. The noise in the shop was like being inside a guitar! It only affected one belt- which proved to be nadgered (permanent twist through long storage in a plait). This is what you need to do to get a good long life vee-drive:-
Use a new belt(s), make sure pulleys are in good order & run true on diameter and are in line; use a straight edge over the pulley faces. Get the correct tension. Find distance between pulley centres & divide by 64. So a 16" (400mm) centre distance would give 1/4"(6mm). At the right adjustment, you should be able to depress the belt at its center by that amount with a hard push with your thumb. DON'T pry the belt on with a screwdriver, you'll damage the cords, or worse damage the pulley rim, especially cast iron. Fit it loose and then adjust the motor (or w.h.y. device) to the correct tension. I used oak wedges and fitted bolts as it's a home-brew.
Run the machine, and after say 24 use-hours, when bedding in will have occured, recheck/reset the tension. Make sure everything is tight- but not to the point of breaking the bolts! If the belt flanks or the pulley bottom(s) look very shiny- they're worn out.
If it still vibrates- suspect fire-damage to layshaft or saw bearings.
Hope this helps,
Alan
