Small fans that have high RPM or the wrong type of bearing for the orientation the fan is used in can get to be noisy.
When you can’t just swap out and use a larger fan with lower RPM you can make the fans you have quieter by using different methods of mounting. Grommets such as the these parts by Aearo Technologies (A 3M Company) work to reduce noise generated by the transfer of vibrations from the fan to the structure you are mounting it to.
Here are some illustrations from the manufacturer of the products attaching to some fan housings.
Alongside hardware to help mount the fan the orientation of the fan and the bearing type that you have can make a difference in noise.
Ball bearing fans can be mounted in any direction.
Sleeve bearings should not be mounted horizontally as it will increase noise and reduce the life of the fan.
Speed is another consideration when bearing type is looked at. When you have low speeds sleeve bearings can be low noise - the faster you go however they generate more noise. Ball bearings are usually the better choice for higher RPM / smaller fans if you need low noise.