I have never really worried about air pumps - I pretty much have used whatever I had or could find. The sound has never been of much concern for they are in other rooms.
I now have to sleep guests in a room that also houses 5 tanks. I had a particularly old and noisy air pump. So I replaced it with a Rena 200.
All I can say is ........... WOW. That is a really quiet pump! I am truly impressed. I did not think there really could be too much difference between the different brands - they all use the diaphrams to pump the air and diaphrams cause noise.
Not in the Rena.
Now that is an air pump!!!!!! I think I shall have to slowly replace all of mine......