I have this same server. Over on OCAU it has been discovered that if you disconnect the front temperature sensor, the machine quietens right down. Personally I have not done this to mine as mines running under ESXI (and is therefore silent except when I have iflicks doing work on the mac os VM). Because I upgraded mine to a xeon, I can run more than one OS and pass through a disk controller (Dell H310 flashed in IT mode) to xpenology. So I have xpenology for the nas side of things, and mac os el capitan so I can share video to an apple tv 2 (no plex for this model apple tv which is a shame), and transcode it without having to cook my poor macbooks little i5 cpu.
before pulling the temperature sensor I'd be placing a thermocouple/digital thermometer on the heatsink and keeping an eye on how warm the cpu gets with the sensor connected especially during plex transcoding (play something while your not actually watching it, and monitor the thermocouple). you can also monitor this in iLO
Then do the same with the sensor pulled. if it gets too hot for your liking just stop the transcode, shut down and plug the sensor back in. These servers seem to have rather agressive fan profiles, and sometimes make a hell of a racket for no reason at all. Yes I agree they do sound like a jet taking off when heavily loaded/starting up.
And yes my first post on these forums. So hi everyone