When car is on whether
Could be lifter noise...valve cams, parts of the engine, are supposed to move the valves up and down to let in air and let out exhaust...there are lifters that good between the cams and the valves to make sure the valves are tight against the cams, they maintain the tightness even when things wear...when the lifter stops doing its job, it lets a gap form between the valve and the cam, and a tapping is heard, actually metal hitting against metal...this could be from the lifter being damaged, dirty, or starved for oil...make sure your oil level is right, make sure the oil is clean...sometimes a dirty lifter can benefit from a fresh oil change...if the tapping keeps up, damage can occur to the valve ends or cam shaft...you probably wouldn't notice the engine not running as well, unless damage occurred that caused to valve to stop working...there are lots of other sounds from the engine that are bad, like persistent knocking, which is a piston in the act of breaking, and not so bad like ignition knocking from using too low an octane gas...depending on the engine type, tapping could be lived with for years, and sometimes lifter replacement, if needed, can be fairly inexpensive...good luck...