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Ralph Mowery

In article ,
says...

For the same reason out of balance tires typically only show up at certain
speeds. They have certain speeds where the characteristics of the system
allow it to resonate. Kind of like a tuning fork will resonate at a
certain frequency. You can fix it by placing small weights, like maybe
one or more dimes, on one of the blades by trial and error. There should be
instructions for doing that for that fan or if not, follow instructions
for a similar fan.

I agree that it is somewhat out of ballance. I had a car that a front
tire became out of ballance. At any speed except from about 55 to 60
mph it was fine as soon as I got in that speed range the stering wheel
would shake so bad I could hardly hold it.

It could also be one of the blades is not in alignment with the others.