As usual, I don't claim to be an expert, but I have a Detroit 3-71. Part of the reason they are so noisy is that they move so much air. I've seen an estimate that indicates the exhaust volume is twice what an equivalent 4 stroke would be. I put a muffler on mine and then ran the exhaust into an old galvanized pressure tank I had sitting around. It seemed like a good idea, but the tank was only about 18" in diameter and the exhaust volume blasted all the water out of the top of the tank. If one could run it in a larger tank, I think it could be quieted a lot. I plan to build insulated walls around it to decrease and direct the noise from the blower and the fan. As Dirtbikepilot said, at lower rpms, they aren't too noisy, mine really starts getting loud when I get it up around 1800 rpm, which is what I need to run the generator head and sawmill I'm building.
Detroit engines and parts are cheap and readily available, which was the main reason I bought it. As loud as they are, a lot of diesel nuts (truckers, bus drivers, farmers, etc.) really love the sound of a Detroit winding up, the noise is unique and sort of attractive if you like loud diesels.
The main reason I bought a listeroid though was to quietly generate power when I don't need the Detroit. Quite a contrast, because it is a two stroke, the Detroit sounds like it is running twice as fast as it actually is.
Ray