[HPsquared]

I'd imagine the fluid bearing gets a bit worn over time such that it's a bit "looser", i.e. doesn't hold the fan rotor as firmly in a straight/central position.

Not enough movement to sense by hand, and no roughness, but enough looseness / free-play to allow a kind of resonant vibration or "wobble" when the fan is spinning faster.

Edit: or maybe it's just very slightly worn, has debris in the bearing or something, such that it's out-of-round. I think it's the former though (resonance) as that kind of noise can be intermittent.