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I am 99% certain it is your starter pinion one way clutch.
This is located in the gear that contacts the flywheel when starting the car.
When the engine starts to idle on its own, for a moment the starter is still mechanically connected to the flywheel and it overs spins the starter gear, BUT since the "one way" clutch is there, the gear on the starter spins faster BUT it does not drive the starter motor. The torque can only move "one way", from the starter to the flywheel.
Why is it making sound?
Over time, the grease dries out.
What is the fix?
Simply remove the starter, (only simple if rear wheel drive) take it apart, clean and re-lube the overrun clutch.
-OR- just put up with it - you are not damaging anything and you will find the sound will come and go
-OR- buy a new starter.
It's item #15.
After a long aggravating time with the above “fix” I’m 99% sure now that it’s not anything to do with the starter, clutch or any of that. Still don’t have the answer but won’t quit till I find it. I’m not gonna guess. Enough trial and error and wasted time. I did enough of that when I was a kid. Has ANYONE tried this fix with success?
And If anyone asks, the oil is filled to the proper level between the H dot and L dot (closer to the H Dot). It has Pennzoil 5w-30 full synthetic in it with a Pennzoil filter (I don't think this filter has an anti-drainback valve, correct me if I'm wrong?). Oil change was done almost 4 months ago, since then I've only driven 1,300 miles or so. This odd sound started happening a month ago and seems like timing chain tensioner noise? (Really hoping it's not). I fear it may get worse if I continue to ignore it. It is quite audible inside and outside the car. The SES lightis not on at this time, although work was performed when there was incorrect green fuel injector installed (A fuel injector for a 350z actually..) causing a random/multiple misfire. I had this diagnosed and repaired at the Infiniti dealership. The SES light went off instantly and has not come back in a week, which now basically tells any technician, including Infiniti, that the engine is operating normally and nothing is wrong but something is wrong, you and I can hear it from the video. This only happens on cold starts. This is the 2009 coupe, black, and manual.
I'm going to start with changing the oil with Mobile 1, since I guess a lot of you use it with no issues and switching the oil filter with OEM. Is my baby dying?