It's "erm" or "umm". It's just a speech habit. Many speakers (not just Americans) use this to roughly mean they are 'thinking but still wanting to talk so please don't talk yet, I haven't finished'. It's like a vocal 'please hold' button. But like I said, it's just a habit. And not a great habit to have. If you started using it, you'd sound very, very strange. Best to just recognise it happens, and don't think too much about it when you hear it.

I think you mean "erm"? It's just a sound we make while we think about what we want to say next... "Is there any more cake?" "Erm... I think so"

Hum, I agree with Peachey. I'm not native English speaker, but we use it in my language too, I mean it's just natural, or there's no such a thing in French?

hmnnnnnnnnnnnnnnn i dont know

Yeah you must be thinking of "ummmm"