Antifreeze Leak

I have a 1997 Pontiac Firebird Trans AM with the 5.7 V8 and approximately 198,000 miles on it. I was stopped at a light when I noticed steam coming from under the hood of the car. When I pulled over to check out the problem I was unable to initially see where the problem was. I did notice that it was further back on the engine and not near the radiator. Once it quit steaming I was able to see that coolant was running out from behind the cover over the manifold on the passengers side of the engine next to the dipstick. What could be causing this problem?