I have a 2006 F150 4.2 six cylinder...When truck is started after sitting 4+hrs (cold) it knocks, sounds like lifter noise. Replaced all lifters & still makes noise when cold. After motor starts to warms up noise goes away. I can change oil & start right away (engine is still warm) there is NO noise...only if engine is cold does it make noise.. Could it be oil pump or pressure release valve?? Can release valve be changed without removing timing chain cover????
thanks in advance!

 

For those who may not know, the 4.2L V6 is cast-iron block and heads - a pushrod engine (cam in block) with rocker arms and hydraulic lifters. It might need a new pushrod. Sometimes one will be slightly bent and tap, then the tapping goes away as it warms up and 'grows'. Might be just a valve adjustment. Also could be piston slap which typically quiets when the engine warms up.