Generally the first place to start is to clean the blades, most noise on these units come from an imbalance of the blades, bias all the blades the same direction (e.g. tugged out) before tightening the screws to maintain the balance,
Next up try flicking the summer / winter switch if your fan has one, this is more to reduce what could be the issue, If it goes away here, something may be rubbing in the fan
If none of that worked, as a last check, take the blades off and set the thing to low speed, if the noise is not there, then its either the weight of the blades loading a bearing (replace motor territory) or something funny is going on, and it gets harder
Most of the designs I have come across do not make it easy to open the motor assembly for maintenance, but if it has bushing (the really cheap fan motors tend to) then a bit of sewing machine oil should reduce or resolve it.