I'm unaware of any study done of air gun noise, but from the British championships at Bisley it is quite noticeable that the occasional and now rare 300S that turns up at the meeting amongst the plethora of PCP's is quite a lot quieter. I don't think there is any difference between the makes, they are all at around the same power so the same air discharge, I'd choose by what you prefer. Rifles are definately quieter than pistols and most modern PCP's have a short barrel a little longer than a pistol barrel and the length is made up with a tube which then deadens the sound.
The most noise at home is from the backstop, the best advice I can give is to get the most modern PCP that your like, you will find it a big step from your 300S.
Good Shooting
Robin