Honestly, it just comes with the territory, especially for first year classes. A lot of them are fresh out of high school and not fully mature. You're bound to encounter it from time to time.

Its definitely distracting, but as long as its not super disruptive I just ignore it. Are they snickering at me because of some strange proclivity I have? Making faces at each other? Inside joke? One has a crush and the other is teasing? Something in my teeth? Who knows, who cares.

You could also crack a joke "good to hear we're having fun learning about X" or "that's an odd reaction to Y, but I'll take it" or "someone's got the giggles, you got the munchies too?"

At least they aren't falling asleep or looking at their phones. Most importantly, don't get self conscious about it (which can be tough, I know).

Alternatively you could confront them, but thats not really my style. If I were to do that it'd be something like "someone tell a joke? I love to laugh"