Try revving your car in Neutral or park and see if you can hear it. If you can set your car in your driveway, E-brake on and get some one to rev the engine till you hear it and get in/under the car and see if you can pinpoint the sound. Hell, buy a stethoscpe and listen to you engine in different spots.

 

It doesn't/won't make the sound while simply revving it. It only makes the sound while in motion and accelerating... This is why I'm so confused.

 

Your 100% sure on that? Try slowly revving it. get unter the hood and get someone else to rev it, you might be able to hear it in there, you would be surprised the sounds you hear in there vs in teh drivers seat

 

FYI - the best way I have described pinging to people is the sound of tiny rocks hitting your windshield at speed....

Seriously, my money is on a harmonic rattle of something if you have ruled out a bad lifter. Remember that a lifter going will sound similar and the car will drive fine at first and worsen over time. However a bad lifter is easy to spot/hear and if the dealership didn't detect it instantly, then it is far more likely a harmonic rattle.

 

How would I know if it was a harmonic rattle?

 

Pinging will only occur under load, not by revving it in the driveway.

 

So like I said, this has been happening for atleast 10,000 miles. Should I expect complete engine failure soon? Sad...

 

No. I am more sure than not that it is a harmonic vibration.

 

Lets explain this harmonic vibration.

 

Ok. have you ever had a bench grinder? When you shut it off and it is slowing down at certain RPM it will sometimes vibrate and you know when this is happening. Anything rotating has to be harmonized or else it will vibrate. its how vibrators vibrate. Now think of your engine's crankshaft spinning with those connecting rods and pistons. Engines are so well balanced that there is extremely minimal vibration. However sometimes you do get vibrations, sometimes worse then others, my brother has a chev truck with a 350ci SB and the whole truck vibrates. This is caused by low frequency vibration. Have you ever been in a transport truck when it shuts down? It also Vibrates alot. That is what harmonic vibration is caused from. An unbalanced rotating assembly causeing a high frequency vibration.. I wouldnt attribute this noise to this alone however, something might be loose and is exaggerating the vibration.