After a little more research or surfing the web for more info. I came across a potential solution. Seems the LS series motors are notorious for lifter tap. Based on what I've been reading piston slap and lifter tap are nothing alike but easily mistaken to be the same. I've been around GM products my whole life (not a mechanic) as dad was in car sales. I've never heard that CHEVY means "Can Hear Every Valve Screaming" kinda funny cause it's true. However, clogged lifters or pushrods maybe the cause of the tap/knock. This maybe due to poor oil maintenance. I would assume that if someone has purchased their LS based car/truck new and religiously changed their oil with a quality filter then this bit of info may not apply and you have another issue. However if your like me and bought used you may want to read on.
I've purchased my yukon 124k and sierra 3500 used and I'm the second owner of both. My yukon was driven by a soccer mom with no time think so getting the yukon to on time oil changes is more unlikely then likely and my 02' 3500 DRW was a work truck that could have had proper oil changes but when hauling loads in extreme conditions for 12yrs for 210k miles oil changes should be more frequent and I doubt they were. Both trucks have a tap. The Sierra sounds like the 6.6L diesel on cold start and it's the 8.1L gas. Both trucks with a warm up the taps go away and run as smooth as can be. The taps are clearly on top of the motor in my opinion.
So, the fix is to replace your lifters and rods. I was looking for away to diagnose the problem with out tearing the heads off and came across the process of running B-12 Chemtool into the lifter bores by removing the valve covers, and 5 min motor flush. If you can narrow down the affected lifters the better. Some people go as far as pulling the rockers off and the pushrods out. This allows a better blast of B-12 in the bores. This seems to help if not solve the tap problem on the LS motors. The idea is the B-12 or flush in the crank case (Gunk, Rislone, Seafoam) will loosen the build up allowing oil to run free again. There are several videos on youtube explaining and showing how to do the procedure. I think for approx 2-3hrs (Mostly soaking time) of work and less than 50 bucks in products plus new oil and filter, basic hand tools it's worth a try than sending the trucks to the dealer and pay 100 per hr for them to say it's normal because we don't see a check engine light. ( based on previous TSB's) Or pony up a several 100 dollas to have them spray B-12 on my lifters or tell me we need to tear out those lifters out.
My approach is to tackle the 3500 first. Hit all the lifters with B-12 and 5min motor flush. I'll leave the rocker and rods in place and use the oil ports to run the B-12 down. Also keeping a quart of Rislone in the oil may help as well and prevent build up. I've seen folks use trans fluid to loose the sludge in their motors. Do what you think may work best. I don't know kinda whay I'm starting with the 3500 first.
Why is this a common phenomena in GM motors? Why do we have bad or clogged lifters and when we change our oil on time and use nothing but the best products? A reason could be that the stock lifters have a under size oil holes which is very easy to clog as a good set of aftermarket lifters are have larger holes? I'm not sure how true this is, I don't have any lifters in hand to inspect. Just something I read a few times. Also there is a notion that GM use Mitsubishi for their lifters. Haven't done the research to validate. I'm sure some who reads this will know if true or not.
If none of this works then I'll be going with a heads and cam swap for both trucks and put fresh lifters in. Why not if your going to tear down the motor right?
