3.4 startup engine rattle, what is it?

This morning I got in the truck to go get some breakfast and when I started it there was a brief rattle from the engine before it went quiet. First time since I've had the thing that I've heard this and 20k miles I've put on. The engine only has 90k miles so it's not like it's very old and I run a quality synthetic oil always so what could it be? It sounded like a lifter rattle to me but doesn't make sense because it quieted down right away. Will see what it does again tomorrow morning I guess and go from there, but I am just looking for ideas in the meantime to see if this has happened to anyone else. We just came back from a 677 mile day trip to pick up another dog last weekend and it never skipped a beat that whole trip.

Ideas?

 

Also, it was a bit colder than normal this morning for July, 47 degrees and it had been a cold rain all night so it felt colder than it actually was. Still, I've started this thing in -20 temperatures last winter and it never made a sound. So I'm pretty sure this is an all new issue developing.

 

id spray the acessory belts with some belt dressing if you haven't already... mine made a noise on startup kind of like lifters clicking a bit ago and it turned out it was the belt chirping.

 

might have been a stuck lifter or something (do we even have lifters ) but try running some oil treatment in it for about a week then change oil and use straight synthetic oil