I know something about these camshafts,iīm from Sweden by the way,so my english probably stinks...
Check your camshafts visually,each lobe should not have any flat areas on the "edge" or sharp edges on the outerside,if they do the camshaft is worn...
A worn camshaft has a "harder" sound compared to a lifter..
If your cam i good,just replace the lifters and clean the oilpipe..
There is a trick to try with bad lifters...they can start knocking because of carbon or dirt inside them,but you can clean them IN the engine!,drain the motor oil and fill half motoroil and half atf-oil,let the engine get warm maybe 20min idle or so and change the oil again,this time only motor oil!
Sounds strange? it isnīt,the ATF-oil takes away carbon and stuff really well.
I have tryed this myself and it worked.
It will NOT work if the lifter is damaged,but they are not always damaged just clogged...
I have replaced the cams on one of my mercedeses too a v8 and it is really easy....
I hope this will be to your help!