New Hydrolyic Valve Lifters making noises, Want to clean/clear up its oil pipes!

Old hydrolyic valve lifters making moises, so I have them replaced with new lifters, but still making noises.
So I have the new ones checked if there is oil in the lifters. There is no significant oil in them. I believe the lifters' oil supplies pipes and so on, need to be clean/cleared up for oil to flow better. I'd appreciate any recommendations for solvent or liquid solution used and methods!. Note the oil pump is working fine, with enough oil pressure.

Thank you.