Sorry mate. IMO this is really bread and butter for a team leader.
You’re not children. You don’t need to be “nice” or “mean”.
hey guy
I’m really enjoying working with you. You seem to be amazing at x. I’m really hoping I can learn about y from you.
If I’m really honest though, I just can’t concentrate with the fart noises. I know it’s just a joke and maybe others are ok with it but it really disrupts my flow, every time.