Random lifter rattle

I have a 69 Dodge swinger 340 with small cam, been rebuilt by previous owner with low miles. On start up sometimes the lifters rattle for just a short period of time and sometimes it goes until it warms up. But sometimes doesnt rattle at all. Never have had this issue on a motor before they either rattle or they dont. Also has 70 lbs oil pressure cold and 60 warmed up. Anyone have this problem?