engine sounds

My cars like a month old with only 3000km and it seems like the engine is not smooth and kinda "rattly" sounding kinda like an old car....is this normal? When starting up in the morning especially the muffler kinda rattles but not from looseness it's a weird sound. Even after driving for a couple of minutes the engine, transmission or whatever sounds kinda like an old car. The performance is still not that bad...I still can't figure out why my car doesn't accelerate like it use to when new. The car sounds old...