a904 knocking in park

Hi I am new and hope I am posting in the right place. I was wondering if anyone could help me out with this one. I have a 65 dart gt, 273 4bl, with a904 trans. When I am in park there is a loud knocking sound. I shift into neutral and the sound almost disappears completely. However, when shifting slowly (millimeters at a time) into gear I can hear the knock come back, once in gear it goes away. I have driven the car with an out of wack trans linkage for about a year. I was unaware it was not right.(again I am new to this) It was shifting me into third before I reached 25mph and am afiraid the undue stress on the trans has caused some serious trans damge. I fixed the trans linkage follwing the advice of some old post on this site. I was told the trans was rebuilt less then two years ago. I can't seem to find any info on my problem. Anyone know what this probelm is and how I can fix it myself, my mechanical experince is limited to basics- water pump, leaf springs, rear shocks, fan clutch...if the job is too big for someone of my experince anyone know good mechaincs in the L.A.-Calif. area???