Sounds like there is stickiness on my tire, but there isn't! I noticed starting yesterday my front tire has been making a strange sound. It sounds like there is a small piece of tar stuck to the front tire and it makes a faint noise of sticking and unsticking from the pavement once per revolution. I inspected the tire by hand, and it is perfectly clean. There is nothing visibly wrong with it whatsoever. It's also not melted. The bike rides perfectly. The tire and tube are both very new. The sound happens whether pedaling or coasting. Sometimes on different road surfaces it disappears, but I'm not sure. 

Because the sound only happens when moving at speed with weight on the tire, I can't reproduce it standing still. Unless my ears are broken, it's not coming from anywhere else on the bike. What could this possibly be? Paranoid delusion?
Ken Hiatt mentioned a wet spot between tire and tube, and I'd like to add something that have already caused me the same symptoms: the anti-puncture band.

In general, there is an overlapping zone of anti-puncture where one tip lays over the other. Depending on how they sit against each other, this might cause this noise. In my experience, this is also a cause of puncture, because the edge of the band can "cut" a groove into the rubber and eventually puncture it.

I wouldn't discard, for the sake of completeness, something inside the rim if it is double-walled (sand, nipple, whatever), or perhaps something related to the loading/unloading of some spoke when the bike rolls with your weight over it.

Hope this helps!