Most of the noise comes from the siphons, which are 2" diameter PVC. Other noise comes from the 25 gallon sump box itself, and then there's the 480watt / 5000gph motor that runs the whole shebang.
I went WAY WAY WAY overboard when I built this thing, and I simply don't need it anymore. I'm going with the canisters, as my tanks aren't even heavily loaded. I'm not an overstocker.
The tanks in question are a paltry little 40g breeder and a 55g standard. The 40g houses a little female JD cichlid, a common pleco who is currently about 8 inches long (I KNOW I KNOW, HE'LL GET HUGE AND I'LL GET HIM A BIGGER TANK WHEN HE NEEDS IT, OK.) The 55g houses 4 small RBP's right now, and they'll either turn on each other or I'll re-home a few till I'm down to one as they get bigger. I keep my tanks really clean, and if need be I'll add another canister to each.
Please understand, this is not really up to me anymore. My sump system DID overflow twice. Once because of a power outage during which the tanks back siphoned through the pressure-side pipes, and another time when a big piece of Pleco's cucumber got lodged in the drain siphon of the 40g. Both times I ended up with fish water all over the carpet and dripping into the basement. The carpet was old and smelly anyhow, so I ripped it out.
My wife and I just spent $2000 on brand new carpet, onto which my two tanks will sit. If they overflowed again, I would have to sleep in the garage next to my motorcycle. These tanks WILL be filtered by canisters from now on. In the future I will probably build another sump system and put it in the fish themed bar that I'm building in the basement.