If you are CERTAIN that it is coming from your front wheel, the two most common sources would be that your odometer sensor is too close to the spokes OR your reflector may be catty-whompas.
If it is a sharp ping-ping-ping, then check for broken spokes. Of course, you would probably also notice that your wheel was not true if a spoke was broken.
Finally, check the whole bike for the source. Sounds travel in weird ways on bikes. Sometimes you swear the sound is coming from the front when it is really coming from the back. A mystery sound drove me nuts for a long time until I realized it was the cable lock swinging back&forth under my saddle and knocking on my seat post.
Good luck.