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What year is the car? With the M272, the tensioner and pulleys are well known to go all the time. I'd replace those first if 2010-2011 model. You could also just take off the belt and check for play in the pulleys/tensioner. I've replaced them with as little as 60k on the engine.

I'd replace them both. It's not like they're that much money. If I were you, I'd just get the gates kit from Rockauto and replace the pulleys, tensioner and belt. If one pulley is worn out, it won't be long before the other one goes and it's well known that all three will cause problems at some point.