engine knocking

Noises are difficult to guess at without being able to hear them first hand. Any time you hear an abnormal engine noise check the engine oil level and coolant level ensuring they are correct. 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.