It totally depends on the round in use, the receiver design, the silencer design, etc.
For example, if you used supersonic ammunition with a silencer meant for subsonic ammo, you'll attenuate the sound somewhat but it'll be a normal 'bang'. Some silencers are specifically designed to silence supersonic rounds by making them subsonic, while other silencers further attenuate the sounds of subsonic ammunition.
If I had to pick a particular sound, a silenced pistol will sound like a very loud handclap at a distance.