I have a 1994 GS300 that recently started making a tapping noise towards the rear of the engine. I believe it has to do with the valves either sticking or an adjustment issue. What could be the possible cause and solution to this problem?

its either a bad fuel injector
or a bad valve guide.

feel the fuel rail and see if you feel the tapping there.
if you do most likely it the injector.
if the injector it will:
runs rich stuck open
runs lean stuck closed

bad valve train then
your looking at a timingbelt and head job

if your car is high miles (80+) i have seen them leak out of the cam seal and over time couse wear on the valvetrain/cam. take it to the dealer or buy a hand held scan tool and see what codes it has.