It does depend on the plane. Some of the older A's seem to have nosier recirc fans at the gate. Once you get airborn, it's not that bad. But it is bad enough to run with the L recirc fan off on the ground. Much quieter than the 757's and hugely quieter than the guppies.

Yes, the 300's are quieter.

But I will say, it is really annoying when a guppy guy tries to wear a noise cancelling headset, and all he does is say "Huh?" when you are trying to talk to him.