How noisey is a water cooling setup?

The loudest thing in my PC at the moment is my Samsung Spinpoint hard disk -
which incidentally is pretty quiet. If I upgrade the processor and graphics
card, they won't be cooled sufficiently by the Zalman heatsinks and large,
slow fans that I have now, so I would have to look at water cooling. I know
the pump makes a noise, but how loud is it and what kind of noise - annoying
whine or quiet deep rumble?