Earlier quoted context omitted.
I don't think it is true: I have an almost silent gen8 and I'm not using the built-in raid
I checked with iLO and fan rotates at much lower speed with RAID configuration. I would be happy to run it without RAID, but it's too loud. I've found that solution on homeservers forum, so I guess, I'm not the only one.
Another issue I've seen people report as a reason for the increased fan speed were hdds which didn't report temperature properly to the controller so the controller didn't know whether the drives were cool or hot.
What BIOS/iLO versions are you running?