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Q: Knocking noise

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What can cause a knocking noise and when you put the a/c on it goes away

My car has 128000 miles.
My car has an automatic transmission.

The most common cause for a noise to go away when the air conditioner is turned on is a faulty compressor clutch. There could also be a problem with a belt tensioner, or one of the other belt driven components. It would be advised to have it checked out by a certified mechanic, like the ones available from YourMechanic, to diagnose the noise firsthand for an accurate repair.

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