Well problem not fixed, but tried lots of things so hopefully someone can come up with a possible answer to my problem, as I'm running out of ideas!
The engine is a 2004 Vortec. According to my manual the oil is supplied directly to the main bearings, big ends, cam and the lifter gallery. Oil is then pumped through a metering valve in each lifter up the push rods to the valve gear. I've got one pushrod that squirts loads of oil, a couple that don't seem to have any oil coming out of them and the rest have a small amount of oil (this is all at tickover).
At the end of last year the engine developed a tappety noise from the lh bank of cylinders. We took the intake manifold off and found the spring clip from one of the lifters was hanging around it's pushrod. The pushrod itself was a slightly dented. We checked all the lifters and a couple were stuck so we freed them off and replaced the one that had lost it's clip, put the engine back together and the tappety noise had shifted to the rh bank!! We stripped the engine down again, replaced all the lifters with a new set (and replaced the dented pushrod) put it back together and started it up today. The noise was still there on the rh side. At this point we took off the valve covers to try and see which valve was giving the problem and found the strange variation in oil flow. I also put a screwdriver on each rocker and listened, they all sounded the same. Then I tried removing the plugs and turning the engine over to feel for looseness in the rockers. On the vortec head there is no way to adjust the valves, you just torque up the rocker and let the lifter take up the slack. None of the rockers felt loose. When the valves are closed you can move the rockers side to side a bit but not up and down. Also you can rotate the pushrods but not move them.
I'm thinking I might have an oil flow problem to the lifters because of the variation in oil flow (we did get sea water in the engine oil a couple of years ago so this could have caused a blockage somewhere) and I'm thinking of trying some flushing oil to see if it can be cleared. However, I'm starting to think that the tapping noise is not the lifters. what else could it be? Piston slap? Small end bearing?