So - I had a bad 24 hours.
This
https://youtube.com/shorts/WcBCzrOvNVY was the noise coming from the engine of my 428CJ after 50 miles and I was worried that I'd wiped a cam lobe.
I finally removed the PS belt and the issue went away, so thank F for that.
Frustratingly, I had the PS pump rebuilt (and it seems to work fine still) so this just may an issue with the return line baffle partially breaking off and becoming noisy, but I'll need to check all that out.
If it needs a rebuild, anyone know someone near the Gold Coast that they have used for this sort of thing before?