As an ex aircraft mechanic I know that if there is something either loose or out of specification along the drivetrain that generally translates to vibration. It could be anything from the screws that hold the motor to the frame to a slightly out of balance counterweight to a bearing that has some slop in how it works to something as simple as a bolt holding the upper arm in alignment not being tight enough ( also can be too tight ). There are several good tutorials on maintenance and tune up of your saw. I believe Rick Hutcheson has a good one on his site. Here is the link to it - http://www.scrollsaws.com/SawReviews/DewltTuneUp.htm
DW