I have a little air pump I got at PetSmart and it's quieter than any HOB filter I've ever used.
That said, you might consider an internal filter. They work pretty well in small tanks and are silent because they're submerged (although if you wait too long to clean them they'll get noisy).
It has a small block of filter sponge at the bottom and it pulls water through that and out the port at the top. Has little suction cups to stick it to the glass, fully submerged. It's quieter than a HOB because there is no above the surface water trickling.