So started the motor again today - this time the noise went away after about 10 seconds which is a pretty strong indicator of a partially blocked pickup tube so off she came...

Blocked much? Bitch to clean out and even bigger bitch to get the sump back on from under the car. It *almost* goes but you have to jack the chassis up to make it fit and that smears RTV all over the place - sigh.
Back on, just waiting for the RTV sealant to form a cured gasket for the sump and I will test in the morning. Fingers crossed....