I think I just went through this problem, I have a mag 9.5 and originally I set it up with clear tubing from the sump to the mag. The clear tubing sagged a little in the middle because it wasnt hard. Air would get trapped in the middle and the pump had a hard time getting water back into the tank,--it was very noisy.
The next thing I did was replace that clear tubing for PVC piping, STILL very noisy, it vibrated on the wood it was sitting on.
My solution was to attach the pump as close to the sump as I could, and I put a new dry sponge underneath. Its now as quiet as I think it could possibly be. The only noise from my tank is the water draining into the sump, and I kinda like that
I hope this was helpfull