have you looked at your fan? it might be dying or falling apart and thats why its loud? or maybe its's just got squeaky ball bearings (WD-40 should fix that).
I'd grease it up and run tit for a while...it should not be as loud. if it still is then turn down the RPM's with something like speedfan. Every fan is different and some have natural defects that make them whistle/rattle or w/e. yours just might needs some lubing. I sprayed WD-40 on a few of my fans and they quieted down.
(make sure to do this OUTSIDE the case where the fan is unplugged...so you dont spray WD-40 inside the PC lol)