exact same, anytime I was out of the saddle basically and I could get it to do it when I would torque the front end of the bike. I had tried everything including re-lubing and re-installing my botom bracket I was amazed actually at how simple it was but the owner said it was pretty common and hard to diagnose.

EDIT: I'll bet you some random bike part i have lying around that is what it is.

EDIT #2: Just make sure you get lube on the bottom and sides, anywhere the fork contacts the wheel or skewer.