On Mission 5 Clean House you will kill 2 sets of 3 people. Once you kill the 6th person and clear the 1st floor Price will ask you to proceed to the 2nd floor. Shoot the door out from the stairs and kill one enemy. Make a hole and toss a couple grenades in to kill the second enemy. Your team can now pass without being shot. You need to go through the broken door and shoot the last enemy in the room to the left. Clear the floor and then it should pop.