engine bay knocking noise on 2004 Lincoln Navigator

when i start my vehicle a loud knocking sound can be heard in the engine but goes away not too long after mostly after i start driving but if i keep it in park after starting up it doesnt seem to go away. what could be causing this?

Asked by for the 2004 Lincoln Navigator
First check the engine oil level! After that your engine has hydraulic lifters that automatically adjust the intake and exhaust valve clearance using engine oil pressure when the engine is running. Engines with hydraulic lifters need to have oil changes at the recommended intervals to keep the oil clean and fresh and the correct weight or viscosity of engine oil . A small bit of dirt or sludge can "gum" up the lifters and cause the oil not to properly adjust valve clearances. Sometimes this noise goes away after initial start up or after the engine warms up.
If it is as simple as "lifter noise" try adding an oil additive called Risolene, it is a low viscosity high detergentcy oil additive that helps clean lifters. Of course an engine knocking sound may be a sign of more serious engine lubrication problems that could result in catastrophic engine failure so get this checked out .
How difficult is it to change the lifters and how long should it take?