Yeah, I always cut the filter open when I change it and inspect the magnet really well, I have never noticed anything out of the ordinary as far as metal shavings or wear signs... I might just have to do that again though... even though I changed the oil LAST WEEK

 

Just pull the filter, cut it open, and if nothing is found put a new one on and top off the oil.
About the only thing that could just go away like that would be a sticking lifter.

 

It more than likely a lifter not pumping up.My 4.8 with 126K miles does it sometimes after it has been sitting for 8 hours or more.Sounds just like a rod knock and pumps up a couple minutes later and quiet as can be.

 

I know this may sound stupid, but something caught in the fan shroud could have made the noise. It happened to me a few months ago after a bad storm and I got some pine needles perfectly wedged in there somehow. When I first started up it was all cool, but once the fans kicked on i heard this hideous knock sound. I did the same thing and almost crapped my pants since I only had 34k on the clock. It seemed to disappear after a few minutes and finally I popped the hood when I got home and found some pine needles chopped to hell stuck in the fan shroud

 

mine did the EXACT same thing right before christmas. same noise you described, changed with engine rpm. just like the notorious engine tick, but louder. it didnt go away for about 10 minutes. 4000 miles later and still no problems.

 

not the fan... I have electric fans and they werent on at the time... they almost dont even kick on but once or twice on my way home, its almost all hwy driving.