Why does your Pontiac 3.8 engine when cold makes a rattling noise coming from the top of the engine about 2 to 3 minutes then goes away why is that and can it be repaired?

While the car is parked, the oil in the engine will drain into the oil pan. When the car is started, it is the oil pump's job to feed oil throughout the system. What you describe may be the tappets knocking for lack of oil at startup (Word of caution: DO NOT rev the engine). You might want to change the oil and oil filter to see if that will stop the problem, otherwise the oil pump may need replacement.