I have a Tetra Whisper 40 and 20 on my tanks. They're fairly quiet, but as dvc said, it really depends on what you have them resting on. Mine are on 1/2 inch thick mat of neoprene rubber. They seem to deliver a good volume of air. I've never had a problem driving multiple airstones in multiple tanks with them.
However, I'm not convinced of their long-term reliability. The first one I had became noisier over time and failed in less than 6 months. I opened it up, and some internal plastic parts had broken off. The two I currently have seem ok.
I've also briefly tried the Rena air pumps. They are a lot more expensive, but they don't seem any quieter. And they don't seem to move as much air as the Whispers. Maybe they are more reliable?
Just my experiences. I'd love to see how others have fared with these pumps.
