Some good ideas I'll have to check there, this bike was just built, its only got 400mi on frame, components, a few thousand on the wheels. The guy who put it together is a real good mechanic, he wouldn't drop anything into the frame or bars.

It will do it whether I'm pedaling or not and it doesn't seem to matter what gear combo I'm using

Headset is tight
wheel bearings have ~1000mi on them and were checked and lubed 100mi ago.
I don't use the presta valve nut thingy
bottle cages are tight and not rattling, grabbed them while riding and it didn't make a difference

Is there a good way to check for chain slap?