This is a sound that people make when they can't remember something. It's very similar to "Um":
But "um" is more likely to be used for emotional reasons, like to be polite or to seem undecided.
"Uh" can also be used like "um" but is often used when a person simply can't figure out what to say next.
A: How many are left?
B: Uhh... let me see. About twenty?