Beeping & "Chriping"?

My cat likes to what we call "beep" or "chirp" whenever he walks into a room and sees us. The beeping is sort of a meow with his mouth closed and the chirping is more of a purr that ends in a meow. He sounds like R2-D2 when he's going for it.

I'm a new cat owner so this might very well be normal in cats but I think its fascinating and did anyone know what it means?  

It's excitement. Normally they do this when they see birds or squirrels outside. Some do this at laser dots. I believe it's called chittering.

very common among kitties. both my kitties communicate with each other in this matter. I'm never heard them meow to each other...only to me.

fun fact: my cat Jet chirps when she's in the litter box pooping.