The same thing actually happened to me exactly. When I first got my reburbished ps3 back, it was absurdly loud. I blew out alot of dust from inside and now, while it still gets a bit louder than my old one, it takes alot longer to do so. My theory is the fan dies alot on some ps3 models (like my old 60GB model) so when Sony receives them and fixes them, they may put in either a crappier fan or a gear to make the fan work harder so it doesn't break again. Combine that with the ps3 design in that, like a PC, it gets alot louder depending on the room temperature and quiets down once the system innards are at an ideal temperature again. Basically, I'd recommend holding onto the PS3, unless the noise never lets up. Sorry for the length. Hope it helped somehow.
I'm actually kind of satisfied with Nintendo now. New Metroid looks great, Galaxy 2, Darkside Chronicles covers RE2 and Code Veronica which is great, and theres still Muramasa out there. Much better than last year. Nice job