dang it, could find the certified machanic's post but this one is pretty close and makes sense...
I recently ran into this same knocking/ ticking issue coming from my 2005 Suburban, 5.3 Vortec. After countless hours of research, multiple engine flushes, oil change, air filter, valve adjustment...etc etc etc...my solution and problem were relatively simple. If you look at your oil pressure and it reads anything under 40 at cold start idle this means your oil pump screen has one of two things: 1) It is clogged and needs to be cleaned, these things are pure metal so a costly replacement wouldn't be worth it, a simple clean will suffice. 2) The oil pump screen o-ring is allowing air into the system keeping the motor from efficiently moving oil through all rockers and rods. Both fixes require removing the oil pan, roughly 10 bolts, and the oil pump screen, 4 bolts. I flushed the engine before draining the oil, replaced the oil pump screen o-ring, cleaned the screen, and installed a new oil pan gasket all in a couple of hours. Low and behold I haven't heard a single tick since doing this, even in cold weather starts, and my oil pressure went from about 30 psi idle to around 45 psi. Take note I had already checked the valves to ensure there they were tight and this may not fix everyones problems but its an easy fix to rule out a few obvious things. One more key note I would like to add is that these motors are KNOWN for breaking the exhaust manifold bolts, typically on the backside of the motor near the firewall. This type of exhaust leak can also give the same noise as a dead lifter when under compression. I had one broken originally and broke another removing it. IF you decided to replace the gasket and pull the exhaust be sure to buy new bolts as the ones they came equipped with are very brittle. I hope this information helps out as many as possible. I spent hours and hours staring at my motor, computer screen, Haynes manuals, and such before finally figuring this out. I have basically compiled a few weeks worth of research as well as trial-and- error on this post. Good luck to all the Do It Yourselfers out there!