1997 Chevy Camaro Knocking from rear

Hey, I am usually good with cars but being in Chicago and it's like 0 degrees I didn't want to tinker with the car. My problem is when I accelerate I hear a knocking sound coming from the rear of the car. It sounds just like if you were tapping on the trunk. It's only at a certain point of acceleration that it will do it. If you push the pedal down a little more or release the pedal it stops. I really am not sure what it is. I bought the car just before last summer. My guess is that it's the gears in the rear end. Just my guess I don't know. Please help before it gets any worse. Thanks alot. This is a great site you have here. Very helpful and convenient. Thanks alot!
Kevin