Sometimes it's worth throwing a baffle on the back side of the cabinet to dampen the noise. I can't tell whether yours is open in the back though. I usually use the pink Owen's Corning foam sheets from Home Depot. You can cut them to fit around odd objects/plumbing. I usually just set mine behind the stand and lean it against the stand a little. If you want to go further you could attach it to the stand to eliminate gaps.
The downside is you won't get as much gas exchange in the sump. I usually run my skimmer intake outside of the stand for this reason. If you want to add more air exchange, a small air pump should help. With mine just leaning against the back though, I haven't had a need for an air pump. There are gaps and my skimmer still draws air from outside.
