I had the issue in a 318 I was running, and also in a 5.0 Ford that I overhauled for a customer. It went away in both engines. Hydraulic lifters. I have since pulled the 318 and installed my 340. I still have the 318 as a spare engine. If I were to chase down that noise, I would pinpoint it with a stethoscope, like the op. If it were in the valvetrain, I would pull the valve cover(s) and "wiggle" each rocker by hand. I am assuming you have the non adjustable valvetrain. I would also strt the engine and visually observe the valve train looking for the excessive lash. Then start eliminating things, if it isn't a lifter, it could be a bent/short pushrod, I am thinking anything other than that would present worse symptoms, for example, I once saw a pushrod break through a rocker on a high mileage Small block Chevy, but that engine symptom was bad idle and popping through the carb. If a cam lobe was worn or incorrectly machined, it should run bad, like a dead cylinder. If it is a tick in the valve train , it most likely is excessive valve lash.