I was messing around under the car today - getting some photos to upload here. While I was there I tied stuffing/wrapping rag around different parts of the headers. Also, I decided to try the “hose” trick to locate the sound. My lack of hands made the hose trick difficult so I ended up using a long funnel like a listening trumpet.
From all of this my new theory is I have leaking manifold gaskets… I hope, because I went and bought a set of genuine gaskets to install. If I get the lifters replaced with enough of the weekend to spare I’ll replace the manifold gaskets.
Here’s how I came about my latest theory; I wrapped rag around where the manifolds meet the cylinder head - to capture any leaks there. The mating surface of the cylinder head is hidden behind the flanged plate of the headers, so I assume any signs of leaks would be hidden also. The rag wrap did seem to quieten the hissrattle, but I couldn’t repeat the test on the rear bank - heat, and getting my hand up in there proved too challenging in the limited amount of time I had free today.