1999 Nissan Sentra Loud engine noise

I have a Nissan Sentra GXE 1999. Whenever I hit the gas pedal, my engine sounds like a lawnmower engine. Its very loud, however I notice no power loss from my engine. I guessed it maybe the oil, so I bought a quart and still no help. The car works fine for now I was just curious what may cause this problem. The car still gets me around but if I can or if its affordable, I would prefer a normal sounding engine.