Mercedes c300 2011 4-matic makes a swirling noise at 40+ mph. I was determined that it was not the wheel bearing or the fan belts. I think is resonance from air entering in but don’t know where
First, enlist a helper.
One of you will drive, the other will move around in the car and listen, to figure out where the sound is coming from.
Be careful if you stick your head out of window!
Air resonances can definitely make weird noises. Can occur if even one window in the car isn’t sealed perfectly. Ask helper to push lightly on the various door windows as you drive, listen if anything they do makes a difference. Be sure to turn off as much as possible that could make a sound during this test. No radio, no A/C blowing, etc.
The next step would be to place tape where the windows should be sealing, listen if that makes a difference. Test the tape first to make sure it removes easily & doesn’t mar the finish.