There could be several reasons for the noise: high-capacity hard disks (they simply make more noise), vibrations transmitted from the disks to the enclosure.(alternative noise buffers from the hardware store could help here, glued under the enclosure) or bad fans built into the device, which possibly rotate faster due to temperature increase.
I suspect it's because of the huge disks, which can't be as quiet as models with smaller capacity. Since the MyBook Duo has a net capacity of 18 TB and can never be inexpensively expanded, I would suggest the following:
Buy a real NAS system, e.g. a Synology DS418 NAS with 4 hard drive bays that are noise decoupled (reduces vibrations). The DS418 has two whisper-quiet fans that can also run faster, but would only do so in extreme cases.
Synology DS418For hard drives I recommend using 6TB WD RED hard drives, they are very quiet in operation and with the 4 hard drive bays you can also set up a RAID5 with 18 TB, plus the additional protection against hardware failure that the fourth drive provides.
WD Red Plus (6 TB, 3.5", CMR)A Synology DS418 with 4 x 6TB hard drives is virtually inaudible in normal operation.
I would try to sell the WD MyBook or return it if still possible.