On the older machines with DC motors, I found that in the case of vibration, cleaning the brushes and armature usually solved the problem. The brushes are located at 90 degree increments around the motor housing under rubber caps. Keep track of which hole the brushes came out of. Blow them off with clean dry compressed air and then clean the armature with a pencil eraser. If it's the problem, the eraser will come out black and sooty. Blow thru the brush holes with compressed air, but try not to breath the dust.