Why is my AC loud when I turn it on?

Match the noise to the part. A squeal for a second or two as the AC engages is a glazed or loose serpentine belt slipping on the sudden load — cheap and worth fixing before it snaps. A metallic grinding or rattle from the compressor itself means the bearing or internals are failing, and driving on it can send metal shavings through the whole system, which turns a compressor job into a full flush plus condenser and drier. A roaring, buzzing or leaf-shredding sound from behind the glovebox is the blower motor or debris in the blower cage.

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