4.3L engine making clicking sound

noises
#1

when ever i start my truck (GMC 1500 4.3L vortec 2wd) every ounce and a while a clicking sound comes from the engine. its when its in park.

#2

ONLY in PARK?

Engine hot or cold?

#3

it makes the sound when its in the gear, park. the engine does it when its cold and at engine temperature