so with the fan off, give it a sideways push to get it rocking.

See the speed it rocks by itself. That is the resonance speed.

Like a pendulum it takes only a ___tiny___ push to keep it rocking if the push comes at the same speed as the resonance.

So you probably have a slight imbalance that is not an issue expect that the resonance happens to be close to that next to slowest fan speed.

You other option is to change the resonance by making the hanging pipe a little longer or shorter.

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