LS series engine ticks.
If the tick is happening due to a broken manifold bolt, the tick sound would be loudest immediately upon a cold start and eventually go away after a few minutes (not within a matter of seconds.
In this case the tick begins half a minute after the engine is running. This would indicate the lifter has good oil psi inside both its "low pressure" and "high pressure" chambers.
The tick sound goes away after a short time has passed and then comes back at a lower volume and maintaines a steady tick.................Hmm................I would say the lifter checkball is out of roundness.
As the lifter moves up and down, the force of the oil flowing into the lifter causes the oil to pass the checkball and the checkball randomly spins in its seat..........as the ball spins, any imperfection in the ball or the seat surface will create a low oil psi condition for that particular lifter resulting in tick...
Sometimes, a lifter steadily ticks for a minute and then goes away for a few minutes and then comes back and repeates the cycle. This is due to the checkball spinning and the imperfect surface intermittently passing above the checkball seat, temporarily allowing low oil psi resulting in tick.
To understand what I am talking about, you need to understand how a lifter works. LS series lifters are made up of the lifter body, a plunger, a ball, two springs, a retaining clip a roller wheel and needle bearings.
Here's what you can try........and it wont cost you any $$$ at all:
Shut the engine off when the tick sound is happening at it's loudest. Next, remove the valve cover. After the valve cover is removed, loosen all the rocker arms to close the valves and to have enough slack so that you can move each rocker off each of thier respective push rods. Next, use your finger and push down on each push rod. Each rod should not move downwards at all. Each one should be rock solid. The one that moves downwards is the lifter that is ticking.
No doubt someone will say, "Just rotate the engine instead of loosening the rocker bolts to close the valves".
Pshh.....Puhleeze, rotate the engine and let the bad lifter pump up with oil and not be able to find anything wrong.
Anyhow, if you do as I outlined, you will have a real good chance of finding the offending lifter.
If you do find a bad lifter, you may as well go ahead and replace them all because if one goes, the rest are soon to follow.
Replacing the lifters will require the removal of the cylinder heads. Not a big job.......can be done in a day.