
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)
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.