More info is needed. Do you hear it at idle. or only when giving it the gas? Do you hear it standing next to the car when it runs?
Are you sure it's internal or maybe an engine accessory? Have you tried to put a stethoscope, or maybe a piece of hose or metal bar about the engine and tried to determine if the noise is internal to the engine or an accessory? Have you changed your timing chain, if it's an older engine? A worn timing chain rubbing its cover can make this sort of noise. Oh well, just some ideas. Good Luck!