Why does my car engine roar?

I have a 2000 chevy malibu. When I accelerate the engine noise inside the car gets loud, much louder than it has the years I have owned it. It does not make any roaring sounds when parked or when it is in neutral. It is a roaring or humming sound that seems to increase in loudness as the acceleration goes up. The RPM's remain where they should be and there aren't any warning lights on. Any ideas on what is going on?