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I love the metallic-edged whine of a multi-stage axial compressor 
If the blade count and / or shaft speed varies along the stages, you get this nice "detuned" sound from the close harmonics (because of the high blade counts). That's further compounded by having multiple units, because they never spin at exactly the same speed e.g. the multiple turbojets typical of '60s aircraft.
There's a similar effect with constant-mesh gearboxes, most noticeable with straight-cut gears, of course. Even the gears not "under load" make some noise, so you get a partial harmonic series according to the gear ratios and gear tooth pitches (e.g. "teeth per inch") in the 'box.
If the blade count and / or shaft speed varies along the stages, you get this nice "detuned" sound from the close harmonics (because of the high blade counts). That's further compounded by having multiple units, because they never spin at exactly the same speed e.g. the multiple turbojets typical of '60s aircraft.
There's a similar effect with constant-mesh gearboxes, most noticeable with straight-cut gears, of course. Even the gears not "under load" make some noise, so you get a partial harmonic series according to the gear ratios and gear tooth pitches (e.g. "teeth per inch") in the 'box.
