I have an 04 GMC Sierra 5.3. A couple of months ago it started pecking the first time I started it up everyday. I had it diagnosed as a lifter knocking and was told that it wasn't a big deal and it wouldn't hurt anything.
I decided to get my engine flushed and had my oil changed and it did not do it again for about 500 miles. What seems to be my problem? I have been told that it may be my oil pump or that I could just put a new set of lifters in it. Its getting pretty anoying and is very loud and i'm possitive it's a lifter .

 

Seafoam the engine. They all do that went they first start up. Do a search and you will see.

 

Could be the infamous "GM Phantom knock"