I bet the fans are dirty. Those Portables had horrible filtration, and just about every one of them I see sucked all sorts of crap thru the motor. Dirt builds up on the fans, then the fans remain in balance when the motor is running -- but when you turn it off and the motor slows down, the fans vibrate from the extra weight on the blades. Same motor in that as a Dial-A-Matic--but with an extra (third) fan on the belt pulley. Either way, they're a pain to work on. Not so much a pain; just a lot of tedious work.
~Fred