I'll take a guess that something like a small leaf, small piece of paper, etc. is going through the fan.

 

 

I'd be looking for loose body trim, had a Lincoln Towncar with similar symptoms years ago and I found it by placing duct tape strategically around the vehicle until the noise stopped, then peeled one piece at a time off until the sound came back and found the culprit. In my case it was a section of loose rubber trim around the top edge of the windshield.
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Try taping the intake cowl to the windshield and then recreating the conditions it happens. If it's primarily a function of driving at speed, my guess is that it's a loose trim piece vibrating from the wind. Other possibilities would be aftermarket window vents or hood deflector?