2004 Pontiac Grand Prix GTP Noise Problem?

When I start the car there's this noise and the only way I know how to describe it. Is that it sounds like your inside an airplane. Its always constant, it never changes pitch. I thought it was the supercharger. But today as i walked around the car I found it coming from the middle section of the car. When I walked to the front of the car you can't hear it,same with the rear just the middle part. I hear it both inside and outside the car. If anyone can help me it would be greatly appreciated. Just brought the car a couple days ago so I don't really know anything much about it.