Eventually tracked it down to the exhaust silencer box. Having stripped the box out to check the baffles were not loose I discovered the car was originally fitted with a MEX 1107 through exhaust (you would have not known as it wasn’t particularly noisy). I changed the box to a MEX 1105 and it instantly fixed the problem. Nice and smooth since without that annoying resonance.
Worth getting down and checking the little metal plate with the number on. See pic below.
Those nice thoughtful people managed to fit the silencer with the plate on the top surface. It is a MEX 1105. I do have a selection of Morgan exhaust boxes so I may try swapping it over if I don't find the source.
I did clamp a stethoscope probe to the exhaust and couldn't detect it as a source, also clamped a probe to the bell housing and again didn't sound like the source, and I believe it is a single mass flywheel so no DMF issues.
The most 'noise' seemed to come from the area of the chain cover, so I don't know if some slack in the chain could produce a resonance.
Thanks
