it's a simple fix. you have to completely loosen the stem bolts first, then tighten the top bolt, then retorque the stem bolts. when you tighten the top bolt, do it only a little, a quarter to half turn. then tighten the stem bolts up and check. continue until there's no more knocking or play. it's a little tedious but you don't want to overtorque the top bolt and ride with the headset too tight, it'll trash the bearings and effect your steering (in a bad way). just tighten it up enough to lose the play.