My experience is that you have a cracked piston, mine does it also and is gone within a minute or two. People I have talked with that know say it is common for the 5.3 engine. Most likely will not cause a problem. Mine has 215000 mi and is still going.

 

I have a 2004 avalanche and it recently started making a loud ticking noise almost like a snap! But it only does it in drive I've tried reversing but no noise and I've tried reving the engine in park thinking it may be an exhaust leak but no noise either! Only does it while accelerating in drive! Can anybody help me with ideas of what it might be? Thanks!