The question is is the noise from the air line, or from the pump itself. A larger silencer will help a lot of the noise is from the airline, but if it is the pump itself, it will not make any difference.
Now with that said, do you have the skimmer/pump on some type of a soft mount? Something like a neoprene mouse pad, or a silicone baking pan? Sometimes that will make a big difference in the noise as well.
Kim