The cooling fan should operate when the AC is switched on. That could account for the whirring.
Does it shudder continuously until switched off? Or just for a moment?
If it's continuous, then the AC belt could be too loose or tight. If it's just for a moment then it could just be the system engaging. Check the tension of the belt and if you don't have the mechanical ability to adjust it then take it to a mechanic.
Good Luck!
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