1993 Chevy Corvette DIFFERENTIAL NOISES

MY 1993 CORVETTE IS MAKING A KNOCKING NOISE WHEN UNDER STRAIN AND AROUND CORNERS. IT DOES NOT HAPPEN ALL THE TIME. I CAN DRIVE SLOW AND EASY AND IT WONT MAKE A SOUND. A MECHANIC TOLD ME IT WAS COMING FROM THE DIFFERENTIAL AND THE CLUTCHES WEREN'T RELEASING. COULD CHANGING THE OIL AND ADDING THE ADDITIVE SOLVE MY PROBLEM? ALSO, THE MECHANIC CHECKED THE CV JOINTS AND U JOINTS AND SAID THEY WERE FINE.