Not a low refrigerant issue - the charge is perfect. I check it every year. Also, I now notice the whistling regularly occurs with the EC activated as well (conclusively rules out refrigerant issue).
I did the air flap self-diagnostic test:
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The whistling occurs with DTC 3. This points to the pod for "Left Blend Air Flap". Can someone tell me where this pod is? If it's on the driver's side as I suspect, then the dash may indeed need to come out.
Does someone have a diagram of all the pods and their locations?