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1998 Ford Expedition - Taps

the engine sounds smooth when i start the car but as soon as i shift to the drive or the reverse positions the engine sounds like a tap tap noise but still runs good…

ford exp ohc motor lifter tap? google it. 21yrs old. xxx miles? frequent oil changes? full service history?
How many miles?

Could be one of the torque converter bolts has backed out, since it only happens in gear. Not sure how to test that theory.