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)
Visible crew/equipment: When they are making the face of big momma we see the camera pass Malcom's back and then out again. When it reaches the other side you can a cameraman with a beard watching him in the mirror just before the other guy walks into the room. (00:17:00)
Continuity mistake: When Martin Lawrence tears the face mask and puts duct tape over it while cooking, it disappears when Mr. Roy comes for a visit just a few minutes later. (00:22:00 - 00:25:30)
Continuity mistake: When we first see Sherry's Mustang, it has a black convertible top, but later in the film when she and Big Momma ride into town the top is now white. (00:04:35 - 00:18:25)
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.