You can have low pressure from bearing wear before you get knocking.
I ran my last 1954 235 that had 15psi cold oil pressure and 0-to-5psi when warmed up. It had no knocks, but most likely it was just a matter of time. I took that engine out because I did not want to gamble on a failure on a long trip.
My 1960 261 with full-flow oil filter holds it pressure at 30psi at all RPMs. My gauge might be off or my pressure relief spring might be different, because other people with tight 261s say their engines stay at about 45psi.
The 1959 Shop Manual shows 35psi @2000rpm I agree with the speculation that you have bearing wear. And, my uneducated guess of probability of which bearings would be in this order: rods, mains, cam. It really does not matter which bearings - the engine is showing significant wear. If you are mostly driving short distances, drive it until it knocks. Let it warm up before revving it and don't push it to high revs.
A rebuild/replacement might be near at hand and you might want a rebuildable block (a thrown rod might bring you to tears).
