95 mustang GT problem

It could be a rocker, lifter, or exhaust leak. I'd personally check them in terms of easiest to most difficult. Look for any evidence of exhaust leaks by checking for carbon buildup around any of the primary tubes near the heads. Even a leak at the collector can be disguised as being higher up, as the sound echoes around the engine compartment. Take a piece of heater hose and hold it to your ear, while probing it around the engine compartment, trying to locate the sound.

Another vote for exhaust leak. This always happened to me when I ran aftermarket headers. Every few months I'd blow a header gasket and it would sound like a lifter tapping.