Cooling fan noise when head is moving in the X axis
As discussed in the comments...

The problem with the fan seems to be its flimsy attachment to the printer head.  The fact the fan chassis is not firmly kept in place allows for it to act as a soundboard, amplyfing whatever vibration nomally occurs in the motor.

You could probably get a fan that is more silent in the first place (noctua is a known brand for silent fans, and it is used on the Original Prusa MK3 for example), but since there's nothing inherently wrong with your current fan, I would simply design a custom, more beefy mount for it.  For added dampening effect you could also use small o-rings as washers for the screws.