I'm no expert, but my hopper did exactly that.
I got one off bazpeeb (TOP BLOKE) and replaced mine, problem solved.
I looked at my old hopper, took it apart, and two bolts that hold the motor in place had worked loose, had I of tightened them up , and reseated the motor, it would of been fine. Looks like someone forgot to put a pair of anti-vibration washers on it.
Baz flogged me the guts of a pukka hopper for £20 inc postage, sweet deal.
Try this first and see if that helps, if not its 'hopper time' hehehe.