Six year old son came home from school making inappropriate moaning noises

I think it’s highly inappropriate and it is disgusting behavior that my six year old son should not be mimicking. 😩

He came home one day and was making very disturbing moaning noises randomly. Asked him where he heard that noise from and he named two “trouble maker” kids at school that make those noises.

How would you handle this situation? He is asking me why he can’t do those noises and I told him because it is not appropriate for a six year old to be making noises like that they sound yucky. And he does not seem to understand. Obviously he doesn’t know what sex is but this is so concerning to me that this is what kids are doing at school. Send help pls. 😕