I had exactly the same problem with a rear wheel. When spokes have been in a wheel for a good while, the points where they meet/cross each other get little dent or groove in them (just like you see in old hubs where the spokes leave the flanges) and the noise is the spokes moving across these dents as pressure is placed on them. It drove me nuts for ages because it was only happening when I was off the bike and spinning the wheel, the only time you can safely inspect it moving, because there was no pressure being exerted on the spokes. I came up with the aforementioned dent theory but couldn't work out which spoke/spokes were making the noise so I tightened them all a quarter of a turn. This got rid of the noise for about 20 miles or so but it came back. I repeated the quarter turn, again on all the spokes, and it's been silent ever since (500+ miles).
I had exactly the same problem with a rear wheel. When spokes have been in a wheel for a good while, the points where they meet/cross each other get little dent or groove in them (just like you see in old hubs where the spokes leave the flanges) and the noise is the spokes moving across these dents as pressure is placed on them. It drove me nuts for ages because it was only happening when I was off the bike and spinning the wheel, the only time you can safely inspect it moving, because there was no pressure being exerted on the spokes. I came up with the aforementioned dent theory but couldn't work out which spoke/spokes were making the noise so I tightened them all a quarter of a turn. This got rid of the noise for about 20 miles or so but it came back. I repeated the quarter turn, again on all the spokes, and it's been silent ever since (500+ miles).