I'm a Used car Dealer more for amusement than as an actual job(Retired Marines / Retired Halliburton).
I purchase a couple 2007 - 2012 5.3's (Silverado's/ Tahoe's /Suburban's etc.) monthly at auctions. I've had cams / lifters replaced when the dreaded ticking starts or purchased with issues. My latest purchase is a nice looking well maintained and clean 2007 Black Suburban LTZ / 2wd / 94k miles. I purchase online now a days. It's the norm here due to distance etc. All pictures were good, no check engine light and actually shows the vehicle running normal (I don't purchase these vehicles with check engine lights on for obvious reasons). Picked vehicle up and it had a failed alternator, naturally the check engine light was on after it warmed up and then it was flashing within minutes. I could send it back to the auction and take my chances or just fix it.
So, I started trouble shooting it myself. Had high misfire count on cylinder 4 and looked like ghost misfire on cylinder 3 which is on the other side of engine after warming up, no misfires cold. Oxygen sensors / MAF working normal, fuel trim short and long -25 with in a few minutes of being reset back to "0" cold or hot , open or closed Loop. Fuel pressure perfect. Compression good on #4. Manually checked intake vacuum was solid and good but would slightly deflect when there was a odd metal sound like a spring letting loose. This odd noise is coming from the cylinder 4 area and is about every few minutes. Did my Google research and found this site. Sent ECM to LT1SWAP and ordered lifter release tool. My question is: do the springs in the AFM lifter actually stick from oil sticking issues or lifter internally failing metal to metal? I had the oil changed but original oil looked ok, put Rislone in it and believe it may be cleaning up the lifters but could be my imagination, it's only been run a half hour or so but misfires / fuel trim issue seem to be clearing up slightly. Fuel trim (L&S)now -18. Misfires down slightly. I assume I re-install the modified ECM and see how it runs? I don't believe I have a complete failed or stuck lifter yet, but likely will? Should I do the VLOM mod if engine runs normal? Thanks for any insight into this problem!
No noise when first started could still work with my theory if oil is thick enough like in cold climate with cold oil. This would only be because the cold oil offers more cushion thus less of a smack sound. If it's not temperature related though considering going down road is fine I would say it's more engine speed dependant. If that is the case and it only does it at idle then I would lean towards a broken timing chain tensioner. This is because the chain will be flopping at idle but only at idle when it's loose without a good tensioner. You might be able to confirm this theory by putting a breaker bar on the main balancer bolt. It's a 24 mm and if you move the engine back and forth you should be able to feel the slop in the timing chain. If you don't feel the slap or hear a tapping of the chain flopping inside there then it's probably okay.
The odd thing is the vehicle makes no lifter noise at all when it's first started (Idles fine) after a few minutes. Then there will be an occasional ticking noise with rough idle , but actually sounds normal while driving it and at the light it will run poorly then straighten out upon acceleration. I sent the ECM in so I can't start it an record this noise but it sounds like a spring letting loose ( AFM lifter letting loose?). I have a Autel Maxisys so I can activate or de-activate these lifters, but there is no noticeable noise difference, but it does seem to make the engine run poorly. I'll know more when the ECM gets back. Received the tool / feed plugs today. That was quick. Fastest shipping I've seen here!!!!
When the lifters fail like in my release video it's metal binding situation and never sludged up. Your situation since the sound is uneven may be a chewed up cam and roller. Basically the noise comes from flat spot on the roller that every so often allows the roller to roll. So when it rolls and comes back to the flat spot it will flop down and make a clack sound.