1 Siobhan: Thanks for the playtest, guys! I’ve learnt a lot about our game design!
2 Martian 1: Thank you. We’ve learnt a lot about invading Earth.
3 Siobhan: Haha, yeah. That’ll come in handy for you “Martians”.
4 Martian 2: It will?
4 Martian 1: Shhh!!

Something to ponder: Would other intelligent species with language have the same sort of shushing sounds that we humans have? "Shhh" is kind of an extended white-noise-like sound, which probably has good properties for being noticeable and being able to be sustained for several seconds, but at the same time not too disturbing or raucous.

A brief bit of Googling reveals that many human languages use "shhh" or "ssss" or "tssss" sounds for shushing. Wikipedia has a good list here, listed under "Hushing".

So perhaps there's reason to think aliens might actually shush people in the same way.