Just my 2 cents here. I had a 89 5.0 LX with 2.02 Aluminum Heads, Holley intake, 320 degree cam ,etc. That Mustang made the same type of sound and came from the #8 cylinder around 5k miles after I made those engine modifications. I know the 5.0 and 5.8 Trinity motors are 2 different animals, but an internal combustion Engine is an Engine. Long story short: I tore the engine apart,cause the sound was driving me crazy too. What I found was when I installed the Aluminum heads, I didn't torque down that particular ARP head bolt as tight as it should have been. Small amounts of coolant would leak by head gasket, into the oil. The oil would foam and the lifters only in that #8 Cylinder would make that
noise. I know, you would think all the lifters would rattle? but it was only the lifters on #8. And just like your engine,it would come and go.I Put engine back together(properly with new gaskets,lifters) and Bingo. The sound went away. If I were you I'd look at the oil & see if there's any amount of foaming or discoloration or signs of antifreeze/water in oil. That could be your culprit. Hope you find your problem. Good luck
Yeah that's a interesting story , good job tracking that down. My issue is always there when hot. never when cold. I hear it through the exhaust also.
I think I wont to try Amsoil just to see if that would make any difference, I just need to find out where to get some.