I made a thread on Anandtech and Reddit about this same exact issue:
I've already shipped it off to Newegg for the RMA. I would rma yours as well, everyone told me to rma mines.
You can contact your vendor if you want to get it exchanged. Or you can convince the name on the box that you need it RMA'd and do that.
Try GENTLY shaking the radiator and card and rotating both. Often, the noise is caused by air bubbles in the rad and/or pump and this MAY reduce the noise.
Sometimes when the pump is below OR above the radiator, sound levels can change, too
I hope mine doesn't have the same problem it is coming Thursday ... RMA of my country Would take Ages and i would be so pissed off !
If it is a bubbly buzzing noise do the following:
While the pc is on, lay it on its side slowly.
Wait a few seconds and lift it back on its feet.
What the noise is - air in the lines.
I have an Antec Khuler H20 620, and get this from time to time. In fact, I think most (if not all) who cool with a closed system get this.