You need to read the Wiki page first.
It actually states that it REDUCES the noise that a firearm makes,that's why they are known as suppressors or sound moderators. As I already stated,if you use the wrong type of ammo(not sub-sonic) the crack of the bullet going through the sound barrier will be very noisy.
Also ,it's possible to hear the metallic noise of the slide cycling when it loads the next round into the chamber.
And lastly,it's virtually impossible to silence a revolver( often seen in Hollywood movies) as the gap between the revolving chamber and the barrel allows hot gas to escape at high speed which is basically a small explosion and is very noisy.
Here endeth the firearms tutorial.