Is it normal that i sometimes make sounds after i fall asleep at work?

When I get really tired, as much as I try to I can`t stay awake. So sometimes when I am at work and I fall asleep, I make a weird sound or say something. It is usually right before I fall asleep and/or right after I wake up.I have absolutely NO CONTROL over it. It is sometimes a word and sometimes more of a moan. Is this normal? What causes this? It`s really embarrassing...