head and cam swap

[gmc67swb]

Here's my problem,was wondering if any of you have ever had this happen...got it all back together started it up and everything was fine after a trip around the block , a lifter was knocking ,ok a little adjustment and all was fine. now the problem is it keeps coming back...have adjusted the lifters 5 times ,on start up its fine no rattle then after its warmed up it starts knocking...new heads ,cam lifters hardened push rods...my thinking is i have a lifter that has a bad spring...if yall have any other idea what could make this happen i sure would appreciate it.....thanks in advance...