Just to finish this off:
- the culprit proved to be the mudguard. Although theoretically a 1950 bike, it has (according to Drags site) a 1951 guard with welded on centre stay pieces. Instead of 1/4 CEI studs and nuts holding it to the forks, the P.O. had fitted stainless bolts. One had come loose and there was a small fracture in the weld of the stay to the blade of the mudguard, allowing it to vibrate just a bit. Just been for a test ride after doing the necessary repair job and the annoying noise has vanished. Although I have no intention of flogging the old girl, I briefly wound her up to 60 mph and it sounds glorious
(it has now got a 23 tooth engine sprocket).
- I am concerned about the tank and would be interested in seeing similar photos to the one attached - I wonder if I have an Indian made one, because with the rubbers fitted as specified, there is almost no clearance to the headstock. I can just get a very thin bit of rubber between the tank top and the bearing race. It is as if the tank mounting points are a couple of millimeters too far back. Also, there is considerably more clearance to the right of the headstock than to the left.
Doesn't seem like I can do much to move the tank back a little, other than "ovalling" the frame mounting bracket holes, which I am reluctant to do, for obvious reasons.
Would be interested in any further opinions.