Ford F-150 (2004-2008) :: Growling Noise That Lasts For A Second Or Two On Cold Start

I have an '05 FX4 with 5.4 and 55,000 miles. I've noticed that once in awhile when starting the truck after it has sat for at least a day I get a type of growling noise that lasts for a second or two. It usually happens shortly after the engine starts up and gets running, but it has also happened after it has run for a minute or two and then makes the noise when I give it some gas to take off. From inside the cab the sound seems like it is coming from the driverside firewall area, but on the rare occasion that I'm able to start the truck and jump out before it makes the sound it sounds like it is coming from underneath the center of the truck. It doesn't make the sound every time on a cold start and never makes it on subsequent starts. I drive truck during the week only drive my Ford on weekends and sometimes it goes longer without being started, but I don't know if this has anything to do with it.

I thought it was a shield vibrating or something but have also heard that the VCT parts can make a rattling sound on a cold start also and this has me a little worried. This sound seems more like a fast growl than a rattle, but it happens so fast it's over before I can really register what it is. One time it made the sound twice, once after startup and again after it had idled a minute or two, thats when it seemed like it came from under the center of the truck. The engine has had a slight cam phaser knock since new, but it is so quiet that it can only be heard if you're inside a building or next to a wall and roll the window down.

Also, I change the oil every 2500-3000miles and only use Motorcraft 5w20 and a 820S filter, and generally baby the truck. What this could be. The truck runs great all the time, I just get this noise occasionally after cold start.