I have had this problem with an old peugeot 106.
It's a regular knock knock knock noise, that occurs when steering, but not at a straight line correct?
If so, it is your velocity joint that has worn. it's the movable joint on the front wheels that allows them to turn laterally independant of the shaft, normally protected by a rubber coating. If the coating is worn, dust gets in, damages the bearings, and causes the knocking. A garage will fix it. not really user fixable, unless you are very good.
Laurence Leach, January 2006