It's most likely a sheared Woodruff key on the crank that's allowing the variator to slip on the crankshaft. It's normally caused by the nut on the variator not being tightened up enough, so it starts to work loose and once there's enough play between it and the crank, centrifugal force throws the Woodruff key out and it gets chewed up. What you can hear is the remains of the key galling on the crankshaft and variator. You'll be very lucky if the crank isn't FUBAR'd by this.
