I hate airpumps. Stupid things drive me nuts. Back when undergravel filters used to be the thing and I was a teenager living with my parents, I had like three or four airpumps running five tanks in my bedroom and I can't even come close to estimating how much sleep I lost because of them.
I tried everything from wrapping them up in t-shirts to tying rubber bands around them in the middle of the night when they started buzzing and vibrating their way towards the edge of the shelf, or against the side of the metal stand. Aurgh! Anyways, putting something soft on the bottom and wrapping it up really tight with something such as a thick rubber band is the best way I found to reduce noise once they get to the point where they vibrate really bad. New ones usually aren't as bad as old ones, though. I've heard the new Whisper ones with the funny legs on them are a decent design. There isn't anything silent, though.
Know there are people out there who feel your pain. I love my HOB filters because they are so much quieter. Same with anything that is fully submersible.