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I would guess the likely problem would be the knock sensor. Sensor should retard the timing due to low octane gas.
Another possibility, given the high mileage, could be carbon build up. There could be so much carbon that the sensor is reading the bottom of the table and the computer can't retard the timing any further.
I would think that you would get a code and a CEL in either event.
I have a 2006 Z51 to and the noise drove me crazy. I found it by accident when changing the interior lighting to LED. It's the rear cargo lights. Pull them out and shake them and see (like I did) if it's the same sound. It's the heat shield - just use aluminum tape and tape the shield to the fixtrue so it can't move.