Gonna sound crazy, but I started noticing an occasional clunk on hard braking about a month back, usually when I sportingly grabbed a fistfull in my driveway coming home from work. First I thought it was the forks, but I couldn't duplicate it by rocking the bike back and forth in the garage, so I didn't sweat it. One day I finally realized the deal - it only happened when the tank was nearly empty. The fast braking made the gas splash forward with a 'clunk'. Knowing this, I could duplicate it anytime the tank was low.