nightrunner

My 2003 F150 with 4.6L Triton V8 starts no problem first thing in the morning but after about 10 seconds I seem to hear a noise from the engine like a flap closing and then it cuts out but if when it starts I then bring the revs.to 2600 i still hear the noise,the revs drop a little but it stays going.It does not do it again all day until it's been left sitting all night and then same thing next morning! Can anyone help please?