Metal Gear Solid 4: Guns of the Patriots Tips

Crawl around the vents in Shadow Moses to encounter Rats. When they see Snake, "!" will appear above their heads, accompanied with a sound. They will run away. Keep chasing them, and they will repeat the "!" alert. Following the rats is a good way to get in. If you call Otacon after you see the rats, Snake will talk about them and how they helped him before.