First I bought a Blueline HD
external pump. It had a noticable hum that could be heard throughout my garage. Was testing the setup in the garage first.
Next I bought a Customsealife Velocity Water Pump- T2. This thing is dead quiet. I don't think I can even hear anything with my ear up against it. It's just amazing. There is a downside to this silence though. With nothing but an empty tank and the pump running for 24 hours, the water temp rose from 75 degrees to 79.
Just thought I'd share my experience.