Putting the airpump up above the tank has helped with noise in the past for me. Like said before, if you hold it and it is quiet try a cushion under it. I good and quiet air pump for not a lot of $ is the Whisper series made by Tetra Aquarium. Cheap and quiet. I have had those same air pumps, the Maxima, and they all got very load over time.
BTW, if the airstone get clogged, the pump works harder and makes more noise. You could try changing the airstone.