Worrying engine knock

[ChrisHallard]

Good evening - I’m looking for advice.
I have a 1935 6 cylinder 14hp. Up until recently the engine sounded like “a sewing machine”.
Recently it has developed a noticeable knock. It would lessen as the engine warmed up but now doesn’t.
Oil pressure 20psi at tick over when warm.
I have a homemade “stethoscope probe” (SU carburettor bowl float brazed to a length of SS tube) and have listened all over the engine.
The knock seems to be coming from the top half of the engine. - listening to the lower of the block it sounds like the sewing machine.
With the rocker cover off - the valves and tappers are like the sewing machine.
But!
If I listen to the rocker shaft pillars between cylinders 2 & 3 the knocking is obvious.

I’m considering my next steps