Someone sends a text message, and someone says beep first. What does that mean? What does DuDu express?

Maybe he likes to call each other "DuDu", or it means kissing, similar to "Yao Yao" and "Bang"; It may also be an intimate code word between them, just an onomatopoeic word, which is cute to type. Haha, maybe he feels like a pig or a small train and likes to pout, so he toots.