Update:
The problem wasn't the pump. It was the silicone tube. It expands and contracts causing a change in the outlet. After 24hrs in the tank, it contracted. If you stretch the tube, it bubbles like crazy. This is seriously the quietest pump out there. I can't tell it's there. Absolutely incredible! Awesome product!

The first day I was blown away. I have it attached to a silicone hose with dozens of pinholes I poked into it, making a bubble wall. The first day It was fierce. The bubbles were raging out of that tube. It was a glorious spectacle. And this thing is whisper quiet. I keep seeing the Tetra Whisper being touted as the quietest in various web reviews. This thing makes that one sound like a freight train. I hear the bubbles coming out of the tube and popping on the surface, but I don't hear the pump. Incredible.
Then I woke up on day 2. The bubbles were at about half what they were the previous day. They don't even make it all the way down to the end of the tubing anymore. There are gaps in the bubble wall. Now, there's supposed to be "cotton" inside somewhere that needs cleaned, but I can't find it. That little plastic circle pops out and theres some kind of synthetic felt disc in there. Is that the cotton? Who knows? They don't explain anything other than it's there and needs cleaned. Now, I did notice fibers from the disc on the white plug. Perhaps that's what happened. The fibers got sucked up? But covering that hole with my hand did nothing to the bubbles. Didn't affect the operation at all. When you remove the plug, it gets louder. But that's what it's there for.
I don't know. Maybe I got a dud. I posted a question on this page, but it's gone now.

Had a very noisy pump for our large aquarium and was looking for something really quiet. This one fills the bill but I wish it had 4 outlets and not just two. So we also have a small pump for one additional bubbler.

The product works as described. However if one port is not used it does make noise.

Works great and it's quiet too I love it . Very happy with my purchase.

This is a super quiet air pump. Even at full blast sitting right next to it you won't hear it. What you will hear is the bubbling water in your tank. I am running two 20 longs with this air pump and it seems really adequate. When I saw up to 600 gallons I thought max power would be crazy strong but its very reasonable. Its plenty strong and I would have no concerns running multiple sponge filters. I'm not sure what the limit is but this could probably run an entire rack of tanks.