The powersupply noise is fixable under warranty. (Easily diagnosable if the sound increases/decreases with screen brightness F14/F15).
The CPU cooling noise is problematic if CPU setting is set to "High". Therefore I never set it to High, except with some games or computational tasks.
To me, the sound level is managaeable for most tasks when set to "Automatic", but if has gone loud from activity it can take a while to quiet down, so in those cases I go to "Reduced" CPU.
That said, I'm happy with the iMac G5, compared to the option of spending a similar amount of money on a G4 (small PB or large iB). No way would I be happy with a G4 as a new purchase in 2005 or late 2004. No wonder PB sales numbers are slipping.