Knocking noise on 92 400e

My 92 400e w/ 70K miles makes rapid knocking noise under acceleration when the engine is cold. When temp gauge reaches above 80 degree, oil pressure gauge drops 1-bar at idle, the knocking noise seems to disappear. The noise started recently after check engine light came on. My mechanic changed the trans boden cable, because that's what the diagnostic code shown, but the problem still persists. Another mechanic suspected "carbon knock" and suggested fuel additive to burn off deposit build up on piston and valves. Any help and suggestion would be much appreciated.