HI Customer (name blocked for privacy), Frightening to say the least, especially since you can't be sure of just what it is. The one thing that would make this kind of noise sometimes and sometimes not. With a different time length in between episodes, would be carbon build up in the cylinders. I don't know if you are familiar with carbon knock or not, but it could scare the heck out of you if you were not familiar with it. I can't promise that this is your noise, but it sure sounds like a good possibility. Normally noise from carbon build up will increase as you accelerate, until you hit a speed where the engine RPM levels off. At that point the sound will either go away or diminish in loudness until you accelerate again. Sometimes, it will just go away. It is really too bad that the dealer didn't get a chance to hear it because they are going to be your best source of information regarding the sound. They do make what is called a "top engine cleaner" or decarboner, that comes in a can. There are a few different types out there, that you could try. Pretty cheap investment and something more people should do for their motors on occasion, even if they are not having a problem. The problem is sometimes it doesn't take but a small amount of the carbon to break loose and go out the exhaust and the noise goes away until it builds back up enough to become a problem again. If you try a can of the pour in type, just read the instructions on the can and follow them to the letter. But be SURE you do it in a well ventilated area and a away from anything that you don't want to have a foul oder on later, because the stuff smells horrid. As I said, can't promise because I can't hear the noise. But I have seen carbon make so much noise you would have thought the rod bearings in a motor where coming apart. The newer cars are better about it because the compression ratios are lower than on the older rigs. But once in a while even a newer unit will develop this problem. Give it a try. If something else happens that may give us any other pointers as to cause please let me know. Would be happy to try to help you cure this problem. If you get it to doing it again, you could also remove the drive belt and see if the noise goes away. If it is still there with no belt, you will have just eliminated all the things that work from the belt as possible causes. You may also want to look under the unit and make sure none of the pans are bent around areas like the flywheel, I have seen flex plates that would sometimes rub on the metal cover and make noise, but not be consistent about it, too. Lots to consider, including the carbon thought. Let me know how it goes. Thanks, Bob
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