The 5D3 is too noisy

Can anyone recommend any method of soundproofing my 5D3 as it is louder than my TV!

The 2 items that could cause noise are the inbuilt Drobo fans or your Hard Drives. Which is it?

The four drives in the 5D3 are:
4Tb WD red
8Tb Seagate Barracuda Pro
12Tb Seagate Barracuda Pro
12Tb Seagate Ironwolf Pro

I can hear the fan working but the hard drives are considerably noisier so I’ve now placed a thick towel underneath the 5D3 and this has absorbed the sound.

It’s not ideal but works.

OK. This is not really a Drobo issue, more a HDD issue, so isolating it on a solid surface (not timber or laminated timber) will also help deaden the vibration.

Your WD Red is likely a 5400RPM whereas your other HDDs appear to be 7200RPM. There could be some oscillation being accentuated by the differing rotations as well. Ahead, possibly upgrading this to a future 7200RPM HDD may also help.