I dropped a couple of sparrows last weekend with my .177 cal Marauder. From where I sat, the pellet impact was alarmingly loud compared to the report of the gun. All I heard was a soft *ping* from the gun. Makes it my go-to gun for morning coffee in the garden.
I have to turn the power way down on my .177 cal DreamTac to be as quiet, and it still needs a moderator as the shroud provides no "hush" at all. Best I've found so far is a BuckRail modular modertor. I won't tell you it is quieter than anything else, but it's as good as my DonnyFL Tanto. But it does change the way the shot sounds. More like dropping a 2x4 on the floor.