The blades are unbalanced. The reason is that the blades of the fan are unbalanced, and the angle of one blade is different from that of other blades. To correct it, this work is more exquisite. If the other blades are unbalanced, the vibration will be more serious. Disassemble the fan, disconnect the blade from the shaft, then support the blade with a stick, rotate slowly and watch while rotating. If any blade is different from other angles, slowly correct it by hand until it is correct. Don't move other good blades, or the more you move, the worse it will be.