Noise from oil filter 1520 f6c

I have a metallic type noise that appears to come form the oil filter. It occurred after removing the engine to paint it. Starter motor, alternator and water pump were removed during this. Valve clearances done. Desmogged, then Engine replaced and appeared to run as before. Changed oil and filter and ran engine all was good.
Bike sat for a month then I ran it and then this noise appeared. Its loud when cold and virtually disappears when hot.
Took water pump out and obviously caught all the oil that came out of the case, so it was almost empty. Ran engine (briefly) and there's no noise (yes yes I know some people wouldn't do that).
I thought it was the water pump but after stripping pump and finding nothing I replaced the water pump.
I filled with oil (10w40), ran engine and noise returned.
Drained oil and very briefly ran engine again, once again no noise.
I changed oil filter to a different brand and I still have the noise.
Oil pressure is 20psi at idle and 65psi at 4000 rpm.
It's definitely not the fabled buzzbar between the frame.
Any ideas?
I don't want to have to strip a motor with only 19k on it if I don't have to.