Odd noise, but nothing found

[agricola]

Noticed an odd constant noise appearing to come from the front of the bike the other day. Decided to investigate as had a window between the humdrum everyday tasks bestowed upon me in retirement. Off with the brakes/wheel etc, bearings are fine and in good nick, likewise nothing else binding, so decided to clean  and check the bobbins while the wheel was off too. All back together ok, so at this stage I'm putting my money on tyre noise. Curious though, I did note that the noise disappears when ridden over a solid metal grid/drain cover etc

[unfazed]

I'd say you answered your own question there. Tyres. Had the same issue with Michelin Road 4s, always a strange whine, when I changed back to Metzler Sportecs and no whine. Like you I removed the front wheel to check bearings and bobbins and all were fine.