The problem is that you have allot of water and air mixing in the pipe and shooting out the bottom of the pipe at a decent velocity. As the water goes down, it is drawing air in to that small hole in top of the durso. There isn't anywhere for that air to go but down and out the bottom. There are a few things you can do that we use to do back in the day before everyone was running siphon lines. First, put a T right above the bulkhead, then add a decent length of pipe going up to another cap with a hole in it, to make what looks like another durso. This way some of that air can escape out of the top of that cap. Next, I see your sump has two bulkheads in it. You can split the line before going into the sump bulkheads and run two identical lines down into the sump. This will slow the velocity of the water down by half, so more of the air has a chance to escape out the top. Finally, after the bulkheads inside the sump, put adapters on the bulkheads, making the line the largest plumbing size you can fit, 2" or better if you can. Then run that line down close to the bottom of the sump. What this does is slow the water down even further, letting more and more air escape through the top of the two pipes coming out the top of the bulkheads. One word of caution though, if something gets stuck in either one of those drains after the sump bulkheads, the back pressure will probably push water up and out the pipe. You can route that vent over the top of the sump just in case, so even if it does get a clog, it will drain into the sump. This all shouldn't cost more than $10-15, and could be done pretty easily. Other than these few things, there isn't much you can do to quiet it down. The mechanism requires it to suck in the air. If it didn't, it would suck in more water instead, causing a siphon, drawing the water down very fast, then sucking air when the water level dropped, breaking the siphon. Doing this over and over. Also, you don't have to have the return go through the back of the glass. Honestly from a functional stand point, it is probably better that it doesn't. If you are worried about aesthetics, you can get black pvc and fittings, and buy a few line loc fittings. I would probably split that much flow up over two line locs anyways. Google furniture grade PVC. If you do it right, you probably wouldn't ever notice it. I know it is a pain, but I would seriously consider converting it to a herbie before adding livestock. Find a few clean containers and hold as much of the water as possible. Those brute trash cans work great for this stuff. We all use them for storing and doing water changes. Odds are you'll end up with a few anyways. A few days of inconvenience vs having to deal with loud drains for the rest of the tanks life is a no-brainer in my book. HTH
