The over/under in my 1655 was doing the same thing. Only thing I could find wrong was the splines on the output shaft and the sprocket were really worn, most likely from mis-alignment because someone didn't use the dowel bolts in the motor mounts. It could be that, a bad bearing or something else. The early over/unders had fewer snap ring grooves in the direct clutch drum, allowing them to wobble and cause vibration as well. 

There's not much that can be done from the outside of the unit, so your going to have to pull it to see what is going on in there. My unit vibrated bad enough that it cracked the bell housing and the plate on the back of the motor.