Since you don't have the service history of the vehicle, I would pull the valve cover and check the timing chain and check/adjust the valves. This is an easy way to eliminate them, and it will take only take you all of about 2hrs(depending on how fast you move). The FSM at the top of the forum will give you the info you need.

If this is your first fuel injected Toyota truck, they do tend to have noisy injectors, whicih can/do make a tapping noise.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed