Unless one of the pulleys is hitting something while turning (highly unlikely) then new motor mounts won't fix it. The front motor mount is it under the balancer (crank pulley)?
If not I can;t see any way the belt or a pulley could bind on something during a turn.
Have you tried to turn the generator and water pump by hand with the belt off to see if they turn smoothly and easily?
I think you have a bearing going out in either the water pump or the generator.
But will be glad when you find the answer.
