Excessive fan noise

The clicking sound suggests that you might have a bad disk in the array.  You should isolate it, and exchange it for a new disk with the seller.  The manufacturer will give you a recertified replacement with a shorter warranty - the seller should give you a new disk.  

One way to isolate it is to power down the NAS, and connect each disk in turn to a PC (using either SATA or a USB->SATA adapter of some kind).  You can then run vendor tools (seatools for seagate, lifeguard for western digital).  The tools might pass, but you should still hear the clicking sound on the bad drive.  That sound indicates a bad disk (even if the data transfers are working correctly at the moment).

If the NAS makes this clicking noise with both drives removed, then it is the a fan bearing in the NAS itself - and you should definitely RMA it, as the fan will likely die an early death.  Again, if it is new, you could exchange it at the seller.  You should be able to get the new one before you send back the old one, so there would be no loss in service.

What disk models did you choose?  Since you care about noise level, you should research the power use and acoustic noise specs of your drives. I recommend NAS-purposed drives (Seagate Ironwolf or WDC Reds).  These use less power than enterprise class drives (which translates to lower fan speeds), and are acoustically quiet. I don't recommend desktop drives, and I definitely recommend against using Seagate Barracuda drives ("DM" models) in a NAS. 

Also, when you first set up the NAS, it can take quite a while for the NAS to sync the RAID array.  The fans will run more during this process, but it is important that you let it finish.  Once it is done, the fan should quiet down.  My RN202 basically can't be heard in my home office during normal operation (though there are other electronics in there that mask the noise).

But if your NAS is in a bedroom or TV room, you might consider relocating it.  If you can't move it to a different room, you could make sure the fan isn't placed at ear-level (putting the NAS on the floor or on a high shelf for instance).  You can also block the direct sound path with something - as long as you don't interfere with the airflow.