I understand that there are different APU's on the market, but I don't know any one of them being significantly louder than others. Most airlines use the APICs.
Your complaints may come from the fact that, due to the size of the aircraft, you spend some time closer to the APU than on a bigger plane. I promise you, an APU on a mid size jet like a 737 or a 320 is about the same.
And for those not liking a running APU, you can happily chose a commuter liner without one, like a Jetstream or a Saab 340 - you will like the cold seats in the morning and the unbearable heat in the afternoon...
Dani