Other mistake: When Nia Long and Martin Lawrence are sitting in the boat on the water, talking about Jascha Washington. You can see a fisherman in the background, but if you look close enough, you can see it's a fake person. (00:55:35)
Other mistake: When Big Momma is taking a shower, you can clearly see that the shower head is a mirror, with the water coming only out of the top, and when Big Momma bends over, you can also see her reflection in the shower head. (00:12:00)
Other mistake: In the scene where they surprise Big Momma for her birthday, Nolan takes the gun from Malcolm (he says give it to me, you ain't supposed to have that, etc. then says I'm gonna go secure the perimeter when he gets the gun) . They show the gun and their hands up close. It is loaded, the safety is off, it is cocked and Malcolm has his finger on the trigger. Nolan is pressing Malcolm's fingers. If he had really been fighting for the gun the way it seems, the gun would have gone off.
Other mistake: Near the end where the real Big Momma knocks over the 2 fighting guys and falls back on top of the security guards, and she's being held down, you can see what appears to be part of a fat suit coming from underneath her clothing. (01:26:50)
Answer: The "draining noises" are Big Momma taking a dump. Big Momma didn't flush either so I'm assuming he took a look at her stools and what he saw made him feel sick.