The Whisper actually has very little noise to start with if you use pellets heavy enough to balance the barrel length and powerplant volume. When gamo endorses their stupid PBA junk, thats when it gets loud. BTW, pellets this light (sub 7gr) will actually break the seals and springs in the powerplant. Suppressors used on PCPs and other numatics will certainly quite them down.
A while ago, I worked on an airgun and installed a barrel integrated suppressor in a sort of way. It used the barrel shroud as the suppressor walls, the baffles were foam, and the barrel ran the entire length. I "ported" the barrel where it runs the last 5" of the shroud. This is a very quite method.