when i start my truck first thing in the morning it makes a tapping/knocking sound after it runs for a few min the noise stops and you don't hear it again the rest of the day. What can cause this.
Older vintage Silverados had a known issue with piston slap. It is worse in the winter but is always present even in warmer weather. The forums have been saying for well over a decade it has to do with the sizing of the aluminum pistons. Way back when things were built to tighter standards, cylinder bores were matched with piston sizes. While all the parts are to spec, some are on the low end of tolerance, some are in the middle and some are on the high end. They used to match large cylinder bores with large pistons and small bores with small pistons. Someone decided that in spec is in spec and it cost more to match the parts. The result is some engines did in fact have piston slap for the first 20 to 30 seconds of operation when cold. I actually complained to the dealer the first winter I owned my 2002 Silverado. It was explained to me this is considered a normal condition. To the point of having me listen to them cold start 3 new trucks all of which rattled. My engine rattled for 16 years and 180000 miles but it started every time.This is not to say that your problem is not lifter drain down issue but the above is another possible reason.
I'm new here, but there's a thing called piston slap. This is when the piston skirt gets worn and starts to hit the sides of the cylinder wall. It happens on older vehicles with high milage. If this is what it is there's not much you can do about it except to just ignore it. Other possibilities are low engine oil level, low oil pressure, worn valve train..post year, make model and milage and it will give more clues
More than likely the lifters are getting worn out. When this happens, they lose the oil in the body while sitting for a long time, then there's lots of excess valvetrain noise. Once the oil pressure is completely up and has completely filled the lifters, the noise will abate.
Replacing the lifters in the engine should completely cure the issue (if this is what the problem is). If you are going to these lengths, you should probably replace the oil pump at the same time. The stock ones start losing the ability to provide pressure. Replacing the lifters in the LSx engines is a pretty big deal, because you have to take the heads off to get to them. This requires a complete upper end gasket set. Since you're going that deep, change out the knock sensors while you're in there (unless they've already been done). Lots of little things which require maintenance and can help the longevity of your 5.3L engine.