Weights, if blades are out of balance.
Usually they get out of balance by getting knocked/bumped.
If a 4 blader, see if opposite blades are inline with each other.
Set to lowest setting, turn off, and watch the tips as the fans slows, and see if any are higher or lower than the others. They can be bent slightly back in place.
If a 5 blader, then measure between the tips of each blade, next to each other(5 measurements), need to be the same, or adjust.