The ammunition is, but the mechanical parts of the gun are still fairly loud.

You can get the sound down to maybe 100 dBA. That’s much quieter than the 160ish that you get without a suppressor. But, that’s still loud. Things that are around 100 dBA include blenders, electric drills, motorcycles, etc. It’s only a very brief sound, but it’s loud for that moment. The Guinness record for loudest finger snap is 108 dBA. So, the quietest gun will be much louder than the average finger snap.

That means those movie scenes where people are having a shootout with silencers in a library and nobody notices, or even in a crowded train station. That’s not realistic. People might not recognize the sound as a gun, but they’d definitely be looking around for the source of that snapping sound. It also means the scenes where someone is sneaking through a house at night killing people without being noticed are not realistic either. If you’re asleep, a loud finger snap might not wake you up, but if you’re awake you’re definitely going to notice it.