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IF your question is about why a solid cam has tappet noise where a hydraulic cam is quieter----------The hydraulic lifter is in constant contact with the cam lobe---solid tappets need to have valve lash clearance so the valves will completely close when the engine is cold, and depending on the cam/engine/manufacture will have clearances ranging from around .010 towards .030--------some companies was around .012 to .016 where Ford was I think .019 on Y Blocks and .025-.027 on the FEs------
So the cam wouldn't slap the tappet, cams were ground with a "clearance ramp", an area of the lobe that gradually took up the clearance over a few degrees of cam rotation--the FEs were destntively louder(so much so that Ford Quickly converted to hydraulics in the 332 /352s very early 1958----they identified these hydraulic engines with a spot of orange paint on the left(I think)side of the timing cover----
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