backplate crushing electronic components
I got in on that deal at newegg. I dont know how loud it is because i never got past installation. There are some design flaws with the backplate that didn't sit well with my motherboard, and i'd imagine it probably wont go together well with a lot of other boards either.
Here's a list of problems with the CM Seidon 120M:
1) the contact area between the backplate and the motherboard has to be clear of components. (see picture) In my case, if i had tightened it some components would be crushed.
2) the backplate is too thick, so you need plenty of clearance (higher risers) in your case.
3) the bolt will grind into your motherboard trace routes so you need to install it with an aftermarket washer.
I ended up getting a Corsair H100i that had none of these problems. I RMA'd the Seidon, even though it's only 25 bucks, because im never gonna use it.
As for watercooling in general - if you are concerned about noise then you should probably go with air. All watercoolers have an audible pump noise. It's not very loud, but it is there and you will hear it if you have low noise fans. You also have to consider the possibility of leaks.. it's just one more thing to worry about.