We recently had a partial failure of the fan in a Synology 218Play, and went to replace the fan. Knowing that the Arctic Cooling F9 fans have a good noise / flow ratio we installed one, however the NAS still kept reporting fan failure in any but full speed mode. Checking on the net revealed that this is a know problem with many types of fans with Synology NAS enclosures, and that the BeQuiet fans do work properly.
Installed a BeQuiet 92mm BL-045 fan (part no XBEQ-BL045).
Result:
No errors from the NAS system, better airflow / noise profile than from the original fan; it now cools completely adequately using the ‘Quiet Mode’ fan profile and is really quiet (the disks make far more noise than the fan!).
If you have a fan issue with a Synology NAS enclosure, the BeQuiet fans are definitely recommended.
(However the rubber grommets holding the fan need to be trimmed to fit.
Also watch out for loosing the rubber mounting grommets as they use the size intended for 40x40 fans instead of the size for 92mm+ fan fitments, so they are too loose and can easily fall out.)