440 head noise. Perpetual problems. Problem solved

The noise is less when you put it in reverse or drive because the engine R.P.M. drops.
It sounds to me like a broken valve spring or a collapsed/damaged lifter or the cam is going flat.
That would cause extra slop in the valve train and explain a few things seeing that you changed the oil supplements...........
You could have wiped a lobe because of lack of zinc in the oil/supplement.