Take it to Wilkinsons, or Bridgestone Bentley or Exleys.

__________________
deadly treadly

mine was also making a simliar sound as to what you are discribing. I pulled it out and stripped it down and turned out to be the locking nut on top of the strut underneath the rubber cap was loose causing a knock. It was a 17mm nut around a hex key. Hope it helps.