I will try my best...the most accurate way to describe it is the sound kids bicycles made when they would put baseball cards on their front fork to get that "click click" noise. I hope that makes sense...
I know that sound - used to do that as a kid. So, the noise increases in frequency with speed?
If you get things caught around the rotor or pads, the noise is going to be more like rubbing, scraping, squealing.
There's the wheel speed rotor and sensor (on the inside of the fork), but if anything came off there the ABS warning should come on.
Could be a bearing breaking up, but they normally rumble and sound rough and I would have thought you'd feel it through the front wheel.
Something completely off the wall - check there isn't a crack in the disc rotor...
, if you can get the front wheel up so you can spin it, that would help enormously to pinpoint where it is coming from and if it occurs in just one place or randomly - but most of us don't have that option.
Bottom line for me, if I couldn't find a reason inside a few minutes of looking, I'd be at the dealers pronto...