Is it a ceiling-hugger, or on a "downrod" ? If downrod, is the rod moving or just the housing ?
Probably the bracket that holds the fan to its electrical box in the ceiling is loose. or the box and it's bracket are loose from ceiling joists.
If ceiling-hugger,
Turn off power at circuit breaker.
Remove blades
Remove motor cover
Inspect bracket screws
Inspect elec box.
Tighten as necessary
If down rod, similar at ceiling collar
See
https://www.google.com/search?hl=en&...YSU&gws_rd=ssl for typical installations