My car has a harmonic humming noise every second or so at highway speeds only. I first thought it was due to my noisy runflat tires, but I replaced them and since the car is much quieter now, I notice the humming noise even more now.

But now that I think about it, I also have the thumping sound at idle that seems to come form behind the dash. It seems to come and go. I want to say that it only happens when the weather is cold, but I might be mistaken.

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