Box fan problem? vibrating uncontrollably?

unplug the fan from the electrical source. Take a close look at all the blades to see if one has a piece chipped off, or a crack in the blade. Then unscrew the knob on the front and take the cage off. Wipe all the blades clean and see if you can wiggle the fan assembly. there is a nut on the front of the fan at the center that hold the fan blades in place, see if it is loose and tighten. Turn the blades by hand and if everything looks good then put it back together and plug it in. Good luck

The blade could be loose or slightly bent.

it sounds like the bearing has gone, not much here you can do, its not worth fixing, its cheaper to buy another, why not go for a slim line tower fan? they take up less room and can oscillate