Corpses do sometimes make a groaning sound when they’re repositioned. That’s because air is trapped in the stomach (possibly after CPR), and when the corpse is moved around (during preparations for removal to the morgue), gravity causes the air to be released, like a burp.
I jumped about 10 feet in the air the first time this happened to me, while doing morgue care after the death of a patient. But I’ve never seen a corpse sit up, or in fact make any movement on its own. (Except, of course, in low-budget horror movies.)
What is your experience?