I have owned one of these sanders for about 2-1/2 years. The only problem I
have had might be related to yours. About a year ago it started making a
clicking sound that seemed to be in unison with the up-down oscillation. I
lifted the sander off of the stand and found a setscrew under it.
Take a look at the handbook George attached to his message. I cannot
remember exactly which screw it was but look at locations for screws
designated #56 that screw into items #53 and #54 (Support Trunnions). Also
look at setscrews #30 that screws into item #37 (crank shaft - two places).
I remember thinking that had the other screw backed out it would have
probably stopped oscillating. Beyond this check all of the other setscrews
on rotating hardware.
Hope this helps