I had the same problem with my hagen air pump, but I also had excess noise from several other things including my fluval 104. I just added sound proofing to the inside of my tank stand. a 4 x 8 sheet is just 45 (canadian) dollars at my hardware store. now i barly hear anything.
(Make sure you have a check valve on the airpump tube to avoid draining the tank in the case of a power outage)
