It's not the chain folks, it's a vibration. The kind you only feel when the bearings are so loose that instead of rotating smoothly they just shimmy around. It turns smoothly when you do so slowly, but get any kind of speed on it and the pulley starts vibrating. Since there's already tension on the pulley from both the chain and the belt, and neither of those are out of alignment, I can conclude that the problem lies with the only point of connection left, the bearings. I'll have a look at them to see if all they need is a cleaning, but it's doubtful.