Hello, I just joined the forum. Hopefully I'm posting this in the right place. I purchased a 2008 GMC Sierra, Z71 with the 5.3 less than two months ago. It has 53k miles. I've noticed it spark knocking, or pre-detonating, mostly when it is in V4 mode, or coming out of the V4 going into V8. It does it at low RPM, runs great if you get into the throttle. No check engine light. The truck is certified used, and still under warranty. I've taken it to two seperate dealerships, and they either claim they can't hear the noise or that it's normal. 89 octane reduces the knock, 93 octane makes it almost go away. I have run seafoam through the gas tank several times, have not run it through a vacuum line yet. It also has a light lifter tick at hot idle, occassionally will clatter a couple of seconds on cold start. I'm not real sure what I should do. I don't want to start throwing money at it, although I did consider changing the plugs mainly to see the condition of the old ones. (only 53k miles, so i'd think they'd be ok). My main concern is the lifters. The light tick at idle is not bad, I've hear worse on new 5.3's, so it's not too troublesome. I just wonder if a weak AFM lifter could be causing the spark knock. If so....I love the truck but may be looking into unloading it. Hate to do that so soon after purchase as I'll lose money. But I don't want to drive a new (for me) truck that I have to work on. Dealer's seem useless if there's no check engine light. And I'm afraid the spark knock may damage the engine. Plus I've read a few horror stories on the AFM motors. Mine does not appear to be burning oil...yet. Any advice, maybe someone's been through this before. I'd hate to get rid of the truck if it was just a MAF sensor causing the problem or something small like that.