Most of the time if you encounter Pastel with a balloon you don't need to worry as she likes to just play with them or keep them around because they make her happy.
But if she ever comes running up to you with a balloon in one paw and a needle in the other it's usually best to cover your ears pretty quickly, because sometimes she also just wants to make them go boom.
It usually results in a very sad Pastel though as she realizes that she no longer has a balloon to play with. But she will usually just find another balloon and do it all over again.

A fun little way in how Pastel differs from me IRL.
While she doesn't mind the sound of balloons popping it's something that absolutely terrifies me IRL and that's why I made Pastel not scared of it as just a fun little difference between her and me.

Art by the amazing  NazzNikoNanuke

Pastel belongs to me  SwedeBrony