It must be tied to testosterone or something

Today I was lying down with Atticus in hopes of his taking a nap, and he burped. Then he burped again. Then it became clear he was trying to burp even more.

Soon his "burps" turned into strangled, gargling sounds, which grew longer and louder. Then he started saying "sy-coh!" followed by more strangling sounds. After several rounds of this I finally caught on and asked, "Are you making motorcycle sounds?" To which he enthusiastically replied, "Yeah!"

So yeah, Atticus is definitely a boy.