I think someone might have mentioned this possibility previously, but I had a similar 'clicking' under power that I could tell was coming from the headset/bars/front of the bike area.

Turned out it was the top cap where it sat on the top of the stem....it was tight, but was rubbing when I pulled or pushed on the handlebars. Putting a little grease on the underside of the top cap/top of the stem took care of it.

To test this out, after you have adjusted the pre-load on the headset bearings and tightened the stem bolts, loosen and remove the top cap, as it is not really needed except to crank in the pre-load on the bearings. Go for a ride and see if the noise is gone. If so, put a little grease on the bottom of the top cap/top of the stem area, and snug it back down again.