If I idle, I hear it: if I'm in drive, I REALLY hear it: speeds up as I accelerate. But the engine sounds great when in neutral. Thought it was engine, could be tranny or differential etc etc etc............got under the truck and have NO idea where it's at. Sounds like lifter noise, but again, at neutral, clicking goes away (while driving if I shift from D to N it takes about 2-3 seconds as the clicking gradually fades away to nothing). Truck runs and shifts great otherwise.

 

There is a plate that bonds the tranny to the flywheel, called the flex plate. If cracks begin it can be a source of the "TICK". In "N" there is less load from the transmission.... suggest your search forum for "cracked flex plate", there have posts with photos. See attached.

 

As can the cam bearings and the oil pump, all can make a ticking sound under the truck.