Ricochet sounds after contact

the sound a ricochet makes is because the projectile becomes deformed and begins spinning in a way that is a high pitches buzz, Passing through an animal after striking a bone or cartilage deforms the projectile. If a hollow point deforms by simply passing through meat it can sound like a ricochet if it exits the animal. tree branches do the same thing.
 
I hear it if there is an angle to the shot and the pellet or slug hits solid ground like decomposed granite or sandstone. Obviously that happens when I shoot up a hill too. I don't like it at all so that is why I make sure to have a good backdrop that is safe and I tend to lower the power of my airguns too. One of the reasons I don't like distance shots into the air is for the same reason I suppose, in keeping with normal firearm practices.