I still can't believe how that 18w DDC isn't giving you high temperatures on your miniNG heatsinks when undervolting
But yea I guess it's possible to dampen the noise, especially at the bottom of the DDC where it's not sealed there is a lot of escaping noise. It's just that by nature the 18w is much louder than the 10w and thus requires more care. So I guess it depends on how much dampening you are willing to build/spend and how practical it'll turn on your rig.
On mine I didn't want the DDC on a bulky isolation box since it'd beat the purpose of compact pump, not to mention I'd have had to kill some hd or cd bay space and would limit mounting options.
Also don't forget that clear top is doing tons of noise isolation for you already, if you run it with the original top it'd be much noisier. I know mine was (DDC 10w). Even the DDC 10w was a bit noisy for my tastes until I bought the acrylic top. So added expenses there.