Valve tap under minor load

Exactly when does it tap? Have you checked the exhaust manifold hasket area, and made sure all the manifold bolts are tight? You could be getting an exhaust chirp under accel untill the manifold "tightens up" with expansion due to heat?

Or it IS possible you have a bit of a piston tap - but that is generally there whether under load or not untill warmed up.

Not likely a head gasket, but definitely check for exhaust leaks.

have the valve settings checked.

Well, whatever the cause...it's killed the truck. Drove it back to my house the other day and on the second to last hill up it started chunking and crunching and grinding. Couldn't get more than 35 mph up the hill, by the time I got to the top I couldn't let out the clutch or it would die. Drove it 1000 more feet and then coasted to my house. I'll open the valve cover on monday and see the damage. It isn't leaking any fluids, so maybe it's repairable? Any suggestions on where/what/how I should check first?