I remember back when watercooling was a new thing in the overclocking world, I read an article where someone was doing convection cooling with his K6-2.. It worked ok, but I think it hung out around 60c (don't all K6-2's hang out at 60c anyways? heh). So it works, more or less, but not that well..
Pumps really aren't loud, and if you pad them well, they're silent. You'll definately run into a hard drive noise issue if you run without fans before you run into a pump issue.