SAN FRANCISCO (CBS SF) — Union Square was filled with noise Thursday afternoon for the San Francisco Municipal Transportation Agency’s 50th annual cable car bell-ringing contest.

The competition began at noon at the plaza, where a motorized cable car has been set up for the contest, Muni spokesman Paul Rose said.

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The event was slated to kick off with an amateur competition, followed by the main contest featuring Muni cable car conductors and grips.

The competition pits last year’s defending champion, Trini Whittaker, against six challengers including the 2011 winner Leonard Oats.

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Whittaker dethroned Oats last year in a “ring-off” after the two tied. It was the first tie in the event’s history, Muni officials said.

The contest will include live music by the band The Slot Blades, and performances by the San Francisco 49ers’ Gold Rush cheerleaders.

The Slot Blades, named after the cable cars’ emergency braking system, is comprised of Muni operators.

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