This is a sound that English speakers make when they're unsure how to answer a question and they need a little extra time to think. For example:

A: Why did you decide to leave your previous job?

B: Well, I... ah... I had been doing that for several years, and I... I guess I wanted to try something new, to challenge myself.

It's similar to "uh.." but sounds a little better.

This phrase appears in these lessons: