99 4x4 100000 mi oil changed every 3000 . Low RPM in gear there is a light tap and it sounds low in the motor. Can really hear it with AC on . Anyone else had same problem ? Thanks.

 

Ford fuel injectors do get "ticky" as they get older.

Lower on the engine could be exhaust leak, it can sound "tappy" even "knocky"

A/C on means more of a load on engine, so exhaust noise could get louder.

Rod "knock" is possible but not likely on a 4.0l OHV, a Rod bearing knock or tap comes from the cylinder firing and the loose rod slapping down on it's crank journal.
If you pull that cylinders spark plug wire or fuel injector wire the noise will stop or get very very quiet, when cylinder stops firing, yes it will miss, but noise will be gone if it is a Rod bearing issue.
Tricky bit is if it is an exhaust leak at the head/exhaust manifold the noise will also go away when cylinder stops firing.

 

Thanks Ron , will check these.