I notice that every now and then in my X as well. Might be alternator noise coming through the speakers that sounds like it's coming from the vents. I used to get that through my aftermarket system in my old car. That usually happens when speaker wires and/or RCAs are run next to power lines. I separated the two in my old car and the whine went away. Not sure how the X is wired, but maybe they didn't keep power wires away from the speaker cables. Or it could be air being forced through the vents, kinda like when you crack your window slightly, the air forced in intensifies as you go faster.

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