It sounds as though at twice the speed of the engine when cold starting then compltely goes away after it has run about 15 minutes. I'm thinking a lifter isn't pumping up. Now can this lifter be changed in the car?

P.S. after it goes away and motor is hot it doesn't do it again until motor sits over night.

 

yes, you can change the lifters in car, just remove the upper and lower intake, then the lifter cover thing thats it.