Might be low AC, causing your AC compressor to cycle on and off. When mine got low, the compressor would kick on then off so hard I felt a thump. Try idling with the hood up and see if the thump matches with compressor cycling on. Also check the serpentine belt and idler/tensioner pulleys.
Hello 650bayarea, I can certainly understand how frustrating these symptoms would be, and am sorry for any dissatisfaction. While I understand you’re seeking opinions, please know that our team welcomes the opportunity to assist you with this vehicle concern. Unfortunately, because we are not certified mechanics, we are not able to provide you any technical advice or feedback. For this reason, we rely heavily on our dealerships to provide the best recommendations regarding repairs. If you have not brought this to the attention of a dealership yet, we’d be happy to assist you in scheduling an appointment to review your repair options. If interested, please send me an email at email@example.com ATTN: Helen with your VIN, mileage, full contact information and name of preferred dealer. Helen GM Customer Care
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