Vortec 6.0 engine knock 2002 Silverado 4x4

I have a question for the 6.0L Vortec Pros out there. I have recently developed a noise in the engine of my 2002 Silverado. It is very interment. First noticed earlier this summer as I was pulling my work trailer to the job site. The Ticking noise went to a loud knocking noise and seemed to resonate from the drivers side of the 6.0 engine. It went away after a few miles and sounds normal again. This has done this to me on about 4 occasions. It starts out as a tick on the highway and gets louder to a knocking noise and eventually fades away to sounding normal again. I do have 221K on this engine and am the sole owner of the pickup from the day I bought it new off the lot. I am anal about maintenance and oil changes at 3000 miles. I am thinking it may be a valve, lifter or spring. I did have the Knock sensor replaced about 4 years ago but that I understand is for fuel mixture and timing to correct for detonation. This definitely sounds more mechanical then detonation problems. I have listened carefully for the sounds at idle and it sure sounds like mid engine drivers side. Any ideas or thoughts about this? Right now it sounds perfect like nothing is wrong. Doesn't burn oil and still has a lot of power. Very dependable still.