Throttle Body Tapping Noise

97 Trans Sport with 3.4 engine has a tapping noise that sounds like a very quiet jack hammer or wood pecker clicking and is barely audible inside the car. Starts only at 1200 rpm and increases to about 1500 or 1600 rpm especially when a very light acceleration is applied. Seems like the motor has to be running 3 or 4 minutes before it starts doing it. Goes away at about 2000 rpm or maybe the road and other engine noise drowns it out. Happens only in reverse or drive especially when shift at about 1200 puts more load on the engine. Is there most of the time. Never happens unless car is in gear with wheels moving. Is very obviously coming from around the throttle body when the car is jacked up and the wheels are moving since I can hear it much easier outside the car with the hood up. The shafts and CV joints appear to have no noise from them. ??