engine and transmission mountings, punit na yung rubber. pag release mo ng accelerator mag-iba ang engine speed so yung engine magbabago ang reactive torque dun sa mountings. since punit na yun rubber may "tok" ka na maririnig at hindi na kasi nada-dampen yung engine momentum. same thing when you release the clutch, dapat sana ma-dampen ng engine and transmission mounts yung reactive torque from the wheels pero punit na. metal-to-metal na yung mounts so "tok" ang result.