My Xbox 360 isn't silent at all (Jasper chipset, so latest model of the fat line), but once games are installed on the HDD, the constant fan noise alone is quite acceptable. At least not some high pitched whine or constantly changing revs. I once wrote a guide how to install a genuine slim 500 GB HDD into the fat lady's caddy, I don't know if I posted that here on the forums too? Anybody interested?