904 causing knocking noise?

Hi all! I just brought a Duster slant 6 back from a 14 year sleep. After much trial and error, she's running. BUT... there is a knocking coming from somewhere. Today I realized that when I put it in gear the noise seems to stop. Another thing is, when sitting still in gear, trans seems to go in and out of gear every several seconds on it's own. Could the knocking be coming from trans or torque converter area? Some onetold me flex plate? Not super versed on transmissions. Any help or suggestions would be helpful. Thanks!