MPG gets emails on fan noise periodically, so here’s summary of issues and mitigations.
Fan noise is often only a portion of the noise that one hears as “fan noise” in an external drive enclosure (unless the unit is using only SSDs, which need cooling, hence the fan). Full size hard drives make their own noise, particularly if they are 7200 rpm drives and there are four of them in an enclosure; the noise can “overlap” in harmonics in some cases, which can be annoying.
In general, decibal ratings for noise are fairly useless because the pitch as well as any oscillations in intensity or unpleasant harmonics can be far more annoying than a steady whir.
Fan noise can be completely avoided with single drive enclosures, such as the OWC Mercury Elite Pro (no fan).
For noise, get a 3 meter or longer Thunderbolt cable, and place the unit under the desk or in a closest or some distance away from right next to your work area. Put some sound-absorbing material around the unit too.
Placed on surfaces like concrete flooring, hard drive vibration is of little concern. But placed on a hardwood desktop or floor, those vibrations can resonate into the material in a very disturbing way (often a low humming vibration). Or not, depending on the material. While the enclosure itself can eliminate some frequencies, certain frequencies are not attenuated and make it through to the surface on which the enclosure rests.
Even the best drives have some vibration, and if with 4 or 8 drives in a RAID, it gets worse. If there is a vibration problem, try 1.25 sorbothane bumpers. They work well to detach any vibrating device from a hardwood floor or wooden desk or similar.
As reader Don H points out (and MPG has used the same approach at times), the closed-cell foam in which OWC drives are shipped works great as a base also.