Another really quiet air pump I have used over the years is the Whisper 60 made by Tetra...WalMart has always had them...these air pumps work great and if you set them on a sponge or rubber mat are super quiet.
One thing to remember when dealing with any diaphragm air pump is vibration is actually how they work. An alternating voltage coil drives a lever attached to a rubber diaphragm pulling and pushing the air. The coil is sometimes adjustable but the noise of the diaphragm remains. This noise then travels...yes, actually travels, through the airline. Now, I have had success quieting the airstream using a heavy duty airline and insulating the air pump, but experimentation really is what is needed to get that pump quiet. After I got tired if the challenge, I just switched over to venturi aeration.