Are the timming chains all way's tight, or is thier slack adjusted in them?
Some times when I start up the truck, it sounds like a chain slipping, than catching, also thier's is sound like compressed air escaping for a brief moment, I took the F-150 in for the axle seals and asked the dealer to look into this and to check the tensinor on the fan belt, as well as the maf sensor, now it takes two three turns of the key to fire up the beast.

 

it is your VCT system. nothing to worry about. it is phasing quickly and rattling. low oil supply to the phasor is causing it. if it is under warranty get it fixed. if not pull the valve cover and remove the VCT solenoid. use a circlip remover and remove the spring and spool. clean and lube them. put back together. if it still does it look into the phasor.

you can also use heavier oil and it might stop.