Poseidon Pumps, formerly known as the "Velocity Water Pump" are for sure most silent pumps at the market. I used to own PS4 model, and it was dead silent. I had to put my hand on it to see if it runs. The only problem is that largest one runs only about 1000 GPH at 5' head. Getting 2 would be an option, especially if you are noise freak as I'm. You may read more about the pump
here.
Cheers!