Let me know what you think.

Thank you in advance

PS: Please leave your opinion on the blog, so we can concentrate the possible solutions on one single source. I'll post here once we have closure to the supernatural issue.

Since the blogger does not work right now, I'll just add a comment here:

I have had exactly the same with 3 front wheels.

Once found a loose spoke - pluck each one to see if it's loose.

The other 2 cases have not been resolved. I have become convinced it is the rim hoop joint making a noise. The hoop joint can be either welded or pinned, and with pinned ones there is scope for a tiny movement which sound will be greatly amplified. This only happens when there is a substantial load on the wheel. It comes and goes. I can even feel the noise when it's particularly loud.

1 loose spoke (most likely not the source of the sound though), very loose headset. Problem fixed. Thank you Jur.