Revealing mistake: When the karate instructor throws the old lady, it’s a stunt double. This is evident because the face briefly changes, and the character’s wrinkles reappear after the fall. (00:43:30)
Revealing mistake: Near the end of the movie, when Malcolm's partner pulls out his gun and Lester shoots him, the bullet hole in the wall appears before the bullet wound in Paul Giamatti's arm.





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.