I have a 2002 Chevy Impala 3.8 litre engine. When racing the engine at 4000 rpm's while in "park", the engine?

sounds like there is exhaust restriction and makes a POW-POW-POW-POW-POW sound. What could possibly be causing this? I need to fix the car this weekend so I will have reliable transportation to school. Please help! Thanks! :o)