Curious if anyone has this problem: My otherwise fine-driving 2003 Subaru Outback with 155,000 miles offers an odd, rapid, click, click, click, click directly from the engine while idling. It goes away once you drive off and then returns when you go back to idle. Not loud, not knocking, just click, click, click, click…My mechanic had no idea…and while it hasn’t overheated yet (knock wood!) the engine always smells like its threatening to whenever I idle for more than few minutes, even on a cold day.
Without hearing the noise, my best guess is that you are hearing “piston slap”.
Supposedly this is not damaging to the engine, but it is definitely annoying.