Frank Krygowski, 2019-02-12 15:40+0100:
I don't know how easy it is to inspect bearings on a
Shimano dynamo hub, but I'd be very reluctant to try
opening the hub until I'd eliminated all other
possibilities. My guess is that your noise originates
elsewhere.

I am starting to think so, and I hope so, because it looks like I would
not be able to service the hub, at least not expecting it to work
afterwards.

Things I might check are wheel reflectors, if you have
any installed. They can rattle against spokes in strange
ways. Check fenders, if you have them; check each
attachment point, and also check that the fender is not
slightly touching the tire. Check any bags or baggage on
your bike. (I've had mysterious noises come from items
rattling in my handlebar bag or saddlebag.) Check
attachment points for all accessories - water bottles
and their cages, racks on the bike, lights, bells, etc.

I'd also advise re-checking the tightness of your axle
nuts or quick releases, your headset, handlebars, stem,
etc. even though it sounds like your problem is
elsewhere. And I'd lightly grease the stem and bar
attachment points.

Will do. I already checked part of these, but checking everything and
greasing attachment points cannot harm. I may even detect some other
problems. :-)

Good luck. It can be devilishly difficult to track down
an intermittent noise.

Yes, quite so, but I like the challenge!

--
Tanguy