Ok, when you get to the Tank Hangar and you've stopped following the rats, stay still. You should see an opening where you can sneak out of. But there is a solider walking around. Sooner or later, he'll stop right in front of you. You should have your tranquilzer out. Aim for his legs and shoot him 2 times. He should fall asleep then. Crawl out of the vent and stand up. Take out your Socom and go into first person view. Just aim at his head and pop a cap in him. It's an instant kill, it works well when you knock down an enemy or put them asleep.
Added by: KingofGreats Feb 10th 2008, ID#8217