[Aerkuld]

I have one, but it's the mechanical spring version. The spring & piston make more noise than anything. By comparison, we have used a sub-sonic .22LR round from a single shot rifle and I think that is significantly quieter.

However, the sound from the .22 is definitely a shot, whereas the air rifle sounds as though someone dropped a pogo stick.

I think the way to go on the Gamo Whisper (if I were to do it again) may be to buy the gas-spring suppressed version, but I don't have any experience of that.