Good morning from a very damp and locked down Northumberland.
I thought I'd start a new post on this one in case it might be useful to someone and I didn't know how to change the title of my "Another Transmission Noise" which was me barking up the wrong tree.
A rumbling noise from the front right hand corner turned out to be the secondary air pump which lives under there and was rumbling away long after it's 90 second run time. This increased with engine speed and could be heard in the cabin. The cause was the outlet valve which was jammed open allowing a built in non return valve to pulsate with fluctuations in exhaust gas pressure. The noise was transmitted back to the pump by the connecting pipe. The valve is accessible on the V6 after draining the cooling system and removing a couple of hoses. It was freed off with release oil and gentle tapping but a new old stock one turned up on ebay ( after I had put it all back together ) so I bought it for the next time..

NB It's a pain to have to remove the front under tray to drain the cooling system and I intended to cut a 2" hole in it to access the radiator drain. Typically I couldn't find my hole saw so that's a job for next time.

Happy is a man with a quiet cat.

Pete