It's so difficult to guess without being able to experience the problem first hand and at least do some basic diagnostic test. Could be fan belts. Visually check the fan belts for correct tension, cracks or damage. Check the engine oil level. Does the noise go away when the engine warms up? On initial start up one can hear a rattle that may go away once the engine develops oil pressure/warms up, if the engine has excessive connecting rod, bad engine crankshaft main bearings or dirty or failing hydraulic lifters (where applicable). On higher mileage engines perhaps that may have experienced overheating problems a collapsed piston sidewall may cause a rattle on initial engine start up that may go away when the engine reaches normal operating conditions.It is safest to get a professional opinion (preferably from someone familiar with your make of car), an mysterious engine noise caught and corrected in time may save costly repairs down the line. Your engine has hydraulic lifters that automatically adjust the intake and exhaust valve clearance using engine oil pressure when the engine is running. Engines with hydraulic lifters need to have oil changes at the recommended intervals to keep the oil clean and fresh and the correct weight or viscosity of engine oil . A small bit of dirt or sludge can "gum" up the lifters and cause the oil not to properly adjust valve clearances. Sometimes this noise goes away after initial start up or after the engine warms up. If it is as simple as "lifter noise" try adding an oil additive called Risolene.