I think the sound of an incoming artillery round would be good. A long dopplering whistle and then a boom.
Secretly replacing the soothing sounds of flushing with a voice saying loudly “I HEARD that!” and “Gaaah, what died in here?” would be effective, I think. And vastly entertaining.
Or a remixed version of this popular hit: “Who let the dogs out! Unnh! Unnggh unghh <plunk>!”