I have a 2003 Silverado single cab, automatic with a 4.8 that I've owned since new. 92,000 miles, 2WD. Synthetic oil since new, regular oil and filter changes. Driven mostly to work and back. About a year ago it started knocking/tapping not on start up but after I put it in gear and backed out of the driveway. Seems to need to be left overnight. Knocks till I pass the neighbors. I hear everyone laughing saying same old problem. Bear with me. I had the pickup o-ring replaced, no fix. My driveway has a slight incline, maybe 15 degrees. Nose up it knocks every time. Nose down and sitting flat it doesn't ever knock/tap. I've tested this a lot. Near as I can tell it's coming from the passengers side front of the engine. I put the brake on and put it in gear, raised the hood and you could hear it. Oil pressure comes right up on startup and stays up hot or cold. Nose up if I let it run until the idle drops off of fast idle it doesn't do it.

 

Lifters Tap,
Rods Knock,
Pistons Slap,
Timing Chains Slap, but it's different from Pistons,
Main Bearings Knock, but not just sitting there idling.

Other knocks/rattles can come from the A/C, Alternator, PS Pump, but they don't quite fit what you're describing.

If you can't tell just what kind of "noise" is coming from it, you might contact a Mechanic, and have him stop by early, or leave it with a Garage overnight and they can crank it in the morning (compliant to the grade conditions you mentioned).

Your description isn't quite enough to tell like this.

 

Left it with a GM mechanic for two weeks while was on vacation. No joy there. He was totally stumped.

 

Ok then...

I'd say drop the Pan, Plastigauge the Rod and Main Bearings, and put a new/set of undersized Bearing(s) in where you had excessive clearance.

I really don't think it's Mains, but as long as you have the pan off...

Have you ever De-Carbed you engine ?
That's flooding it (while running) with a specific De-Carb cleaner or Tranny Fluid, and letting it sit overnight.

 

I ran water thru it but that did nothing. The mechanic I have suggested not using the GM cleaner as he has seen it plug the Cat but that is a simple thing to do. I am trying to find the simple things before having the pan pulled again.

 

Understood and agreed with...
But, I'm running out of ideas here.

Anyone else want to chip in and suggest something ?