From everything I've heard the Rena pumps are supposed to be the quietest.
I bought a cheaper one (PenPlax, and it's in the mail right now) and I was thinking of trying some Dynamat to sit it on. It's a product made for car audio that goes in trunks and is there to absorb sound and vibrations. It's usually about $15 for two 10"X10" sheets.
I have room for my cannister filter and air pump in one area if the cabinet an I'm thinking about lining the entire inside with dynamat. If you have the pump sitting out a small scrap big enough for it to sit on would probably cut down on the sound and you could probably scam a tiny piece from a local car audio shop.
Here is a link to where you can order it from Crutchfield.