maybe check the venturi for a clog, I doubt it's clogged, but I don't know what else it could be. Something else I was thinking about that could possibly add noise is the depth of the pump. 10" seems pretty deep to me. Raise that puppy up as far as you can, who knows.
My thinking is this, on my 5-2, If i'm right next to the skimmer, I can hear it sucking in air. Now you've got a bigger skimmer and pump combination. It's obviously sucking in more air than you're used to. I don't think anything is wrong with your pump. Venturi's are just inherantly a noisy peice of equipment.
The silencer WILL work.
Rest easy my friend.