My iMac is completely silent, the HDD, fans, optical drive, etc all of the time. I'm ripping DVDs to h264 (so there's continuous HDD writing, DVD reading, and the CPU is maxed out) and I can't hear it at all. It's the quietest computer I've ever seen... so much so I'm concerned about the temps and trying to see what the normal ranges are (haven't had any troubles like shutdowns or errors or anything, so I'm sure it's fine).
I think the fact that a large part of the machine is a solid hunk of aluminum helps it dissipate heat well.