ううん (“u-un”; “No”(informal))
This is also pronounced with your lips closed (try saying “ooh-oon” with your lips closed). Whereas うん(un) is pronounced with a falling tone, ううん(u-un) has an rising tone on the first う(u), and a falling tone on the うん(un). Let’s see the example:
It’s tricky to tell the difference at first, but if you practice listening to Japanese, you’ll get the hang of it!