Tunze sells the power socket that allows you to add any pump you want. Something like an aqualifter will be very quiet. If you have a Controller or timer you can also set the ato to only turn on when your away.
I really like Tunze products and have a 3155 on the tank in my living room. The ATO is in the cabinet and the pump noise isn't an issue. However, the nano is in my bedroom with the ATO adjacent to the tank. Maybe it's amplified by the container I'm using, but the pump will wake you up! I tried a second pump with no change.