The belt pulley is loose and shaking.
Hi. The bolts securing the pulley should be inspected for damage because they shouldn't be coming loose. The bolts should be tightened if they are not damaged and make sure that components attached to the pulley are not damaged. Vibrations like these are not good for the engine and this should be repaired soon before something is damaged. A certified technician, such as one from YourMechanic, would be able to inspect the vibrations for you and let you know how to proceed.
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