Hissing

I have a relatively active imagination, so I've spent every night for the past week wondering why there are now snakes living in my toilet tank. At least, that's what it sounds like. And what goes through my mind as I'm showering. Or trying to fall asleep.

The hissing started about a week ago, right after the city installed our new water meter. I've lifted the lid and there's no signs of leaking or continuously running water. Everything works fine.

It just hisses, constantly and quietly.

Should I be concerned, or only if it starts gurgling?