This was my first time doing anything with water cooling, so I had no idea what to expect since "pretty loud" or "quiet" are all relative terms.
I am impressed with how quiet it is. The pump gives off a high-pitched continuous tone, that I (with good ears) can only hear when I level with the pump within a few feet. When I put the pump inside the case, I hear nothing at all, it's amazing.
On the down-side, I tried to adjust the speed of the pump to see how it would effect the system, and the red dial made a loud snapping noise when turned to 4. I quickly turned it back to 3 and haven't had a complaint since.