rocker or lifter noise please help

SHat Shat Shat I just spent 3 hours in the shop trying to find the source of the knock in my 1965 273 motor. It has 600 miles on it (rebuilt last year) with a hyd. cam converted from solid lifters. Ran great all last summer.
It started last weekend cruising at 55, a constant rattle. Feared it was a rod knock, but did not happen under load only cruising speed 2000 RPM.
The rockers are loose some lifters do not seem to stay pumped up.
Oil pressure dropped from 60 to 40psi when hot.

So whats the fix? all new lifters?
Can I put solids on the hyd cam?