Little setback today:
I noticed a very small leak on the gasket leading from the sump to the return pump (external). When I tightened it, I think I crushed the seal a little, and the leak got worse. Had to drain the sump and reseat the seal (coated things in Teflon tape while I had it apart).
Then... After putting the gasket back together, I decided to fill the sump with water to test it. Water went everywhere. I'm standing there like an idiot trying to figure out what went wrong.
As I look around, I realize the return pump is sitting disconnected off to the side. I put the gasket back together and forgot to reconnect the pump.. Doh!
So... Now it's all back together and running, and I've got a washer full of wet towels.
Playing with the sump a little more, I realized a lot of the noise was coming from the spillway leading from the middle to the final return chamber. Two little notches are cut out and water pours from them with quite a bit of force.
I cut a about halfway through in a foam filter block lengthwise and set it over the two notches, so now the water runs through it before hitting the return chamber. It reduced the impact of the water and the sound.
I placed an order for a few of those silicone pot holders, as well as some cheap sound deadening material. I'm going to play with it and see how quiet I can get this thing.