Are really only 2 ways to quiet a PCP:1. Use less air per shot.2. Use an efficient LDC/Should system. Using less air per shot does not have to mean a big drop in power. More efficieny in air use can often be had at nearly the same power. A slight drop in power (like 10%) can often give you a really big decrease in air use per shot...which you will hear as less noise.SLIGHT decrease in noise is often noted with the heavy weight pellets...I do notice it....and they do tend to earn a little more energy (less velocity, but more energy) as well as often give a slightly longer shot count. That pretty much fallls under "less air per shot" too, but it's a slight sound decrease.If you've got neighbors close enough that sound is a major worry, then likly your backyard isn't gigantic. So there just may be no real need for big-power for the use you have.
LOUDEST your PCP will be is when blank-firing with no pellet chambered at all