when i first started it, without co2, it was nearly silent. With co2 it is by far the noisest part of the tank... i have the pump wrapped in foam and i hear the pump "chopping" the co2 across the room. i e-mailed vertex and they got back to me within hours, saying that Proline Aquatics will help me..i will tell you vertex is a great company, they respond quickly and find solutions. i hope they can fix this?