Yeah, I'm using the silencer that came with the unit, but I made one as well - both are the designs that you're speaking of. Mine uses a 2" coupling, the one that came with this uses a 1 1/2" coupling. I figured the one that came with it was probably better...I was wrong. Yes, the water level fluctuates.
I changed out the silencer that came with the overflow box with one that I made myself - it handles more water and I'm getting max flow out of the box now (no need to throttle back on my return pump anymore). It is a little quieter in that every now and then the water rises a little in the external box and it's silent for about 20 seconds, then the water goes back down and gurgles. Right now, the cap has one hole with a tube sticking out of it.
So, dskidmore, you're guess is this means not enough air? So, in addition to this hole with the tube, perhaps I should drill a 1/8" hole next to it? Then, maybe add 1/8" holes until the water level stays even and doesn't gurgle?
Any help would be greatly appreciated. This is my last obstacle in completing this sump.