I may be incorrect on some of this, so feel free to point it out, but i'll put in my .02 worth. My experience with tappet style valves is only with motorcycles, but the same principles should apply. The way i understand it, on a head that uses the bucket/shim system can't get "looser" (with respect to the cam anyway) with wear. If the valve is wearing, the clearance should actually get tighter, basicially, the valve seat is wearing thinner and since the cam rest directly above the valve the valve spring pushes the valve closer to the cam. If this is the case then that means that a valve wearing down is actually likely to make less noise(from less slop), but make the truck run worse since the valves are no longer closing all the way. One indicator on my bike was hard starting, especially when cold, once again due to the non sealing valves, which of course means lower compression. The noise would seem to indicate a valve sticking in the seal and then "popping" back up to seal when the piston started it's compression stroke.
One other thing, I had a 90 that i had a new v6, straight from the toyota factory, and it had a little ticking noise too. Never could figure it out, and the toyota tech said every engine he's ever installed from toyota sounded like that. Anyway, thats my thought for the night.