transmission knocking
 

i have a 99 f-150 a w/4.2l 5-speed manual w/56k miles. my truck runs great but when i put it in first and let the clutch out and start moving slowly it knocks. at first i thought it might have been a lifter or something in the engine but it only does it when the truck start moving while in first and sometimes in second if i'm going slow. just was wondering if anyone has any ideas. i havn't changed the transmission fluid yet but i'm gettin ready to in a couple of weeks. thanks