You’re welcome 
Oh good! I think a vintage bike meeting is the perfect chance; perhaps you can find somebody who can identify the issue locally, with the bike on hand.
Noise not occuring at the workstand - that means no load.
You wrote a consistent sound - it does not depend on pedaling.
But it does depend on speed?
I think that was good advice, better be careful with WD40.
Hmmm…what I would do first anyway is that: Check wheels and frame tubes for cracks.
Please feel free to send videos also to my address above if you’re unable to upload.