i have 80mm case fans that have higher db ratings. the default cooler that comes with the Athlon 2000+ (the thin one) pulls over 72db.
its the ducting. that plastic is vibrating and its letting the noise out the back of the box. FlowFX is a damn nice cooler for free, if you ask me. i think the swithching (turning on and off) intermittently creates annoyance for the user experience, but i accredit most sound complaints to people just thinking its 'supposed' to be loud because they read it somewhere.