Fortunately, I found an explanation for my new iMac’s noise level: I was using the fast user switching support to enable different environments for my wife and I, for some reason her Safari process had gone up to using 100% CPU, and this was not reported in my activity viewer because by default it only shows the current user’s processes.

So: If the box really has a lot to do, the fans kick in and they are pretty loud. Running at normal load, it’s still louder than the Cube, but you’ve got to have a really quiet environment to notice it.