If you have the system on "auto," it will vary the fan speed based upon the difference between the cabin temperature and the desired cabin temperature. At a light, with no air moving over the car to draw away heat from the sun, the AC will have to work a little harder.
If you have it with the little amber light on that says auto it will change the fan speed up and down automatically to keep the car at what ever temperature you have set. Check your sync 3 manual or you can go online and find the PDF of the sync 3 manual and it will explain all that.
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