The typical vacuum system on a car with a leak is (I believe) open at both ends (the venturi & the leak). The flute is also open at both ends with the lowest note (fundamental) at twice the length divided by the speed of sound (2L/v = 25.6" at Middle C). (The Saxophone, which is closed-at-one-end-conical has the same harmonic series). The pitch at 500 would be a slightly flat “c” (middle of treble clef=C5=523.25 Hz). At 500 Hz at the 3th Overtone (4th Harmonic) would give a fundamental length of about 51.5 inches. The harmonic series up that high (the 3rd, 4th & 5th overtone) would produce the tonic, third & fifth that you would hear. Any lower than that, probably wouldn’t be loud enough (insert alto or bass flute sound here…), and any higher than that would be a flattened minor 7th–the energy required to get that high would be almost twice it would be to get to the 5th, and so the engine & road-noise would be so loud that you wouldn’t hear, either.
With a closed-at-one-end-cylinder, similar to a clarinet, the frequency would be measured at the speed of sound divided by 4 times the length (4L/v). This would produce a series that would skip every other partial. (ie: Fundamental, octave+5th, 2octave+3rd, 2octave+(flat)7th, etc.) At approximately 500 Hz, with a purely cylindrical tube, playing the 7th, 9th, & 11th overtones, would give a (fundamental) length of around 45.3 inches. And the Overtones would sound very out of tune… (Flat minor 7th, Major 2nd, and Sharp Augmented 4th)
It could also be a chamber of some sort (with a linear length around 50") where the wind is brushing past the opening (Could be anything/anywhere–the ear cochlea is a spiral to take up the least amount of volume). Could be a hollow bumper, closed-off wheel well, hole in the trunk, etc. All it would take would be the wind rushing over the hole at a certain angle (like blowing a bottle) to produce it.
The Formant is one of the frequencies a system amplifies naturally–such as motor noise, piece of wood, bathroom/stairwell singing, etc. It could also be a loose piece that vibrates at those frequencies, although the harmonic series is much different (similar to chimes): octave= 1.414 times the length (root of 2 - not 2 times the length, as with strings and air).
Since the speed of sound is constant (0 to 10000 ft = change around 3.55%; humidity = change 0.1%-0.6%, etc ), if you could determine the pitch, you could probably find the source.
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