Why does my engine sound like a diesel???

I have a 2007 g6 2.4L 4cyl when I turn my car on It sounds fine on the outside but as soon as I get inside and close my door the engine sounds like a diesel engine I asked my gm mechanic where get all my service done and they said it should be normal but I don't know because Im never in my car without my stereo on lol what about you guys? Any sounds? Even after I'm done driving it still sounds like it I'm kinda worked I only have 52k on it and still have a bumper to bumper warranty but it cost me $50 to get something fixed so I don't want to bring it in for nothing please help!!!!