I strongly suspect that the unexpected doppler shift is from jetwash.<p>That is, the principle source of noise from a jet aircraft isn't the engines directly (turbine spool), or the fuselage's passage through the air (turbulent white noise), but the stream of hugely-accelerated air which has exited the turbine(s) and is now shredding itself against the stationary surrounding air. The noise source therefor isn't a <i>point</i> (engine) but a linear source (the turbulent shred-wall interface between the jetwash and surrounding air), and it is <i>moving rapidly backwards from the aircraft</i>.<p>Which means that as the aircraft approaches you, the jetwash / shred turbulence is moving away from <i>you</i>, and is doppler-shifted toward lower frequencies, and once the aircraft passes minimum distance, the jetwash is streaming <i>toward</i> you, at a high fraction of the speed of sound, and should therefor be doppler-shifted <i>upwards</i>.<p>The insight that it was jetwash and not engines themselves making noise became clear to me when I lived near an airport with a road passing immediately behind the runway. I happened to be cycling past one day as a jet lined up for take-off, heading away from me. I was positioned directly behind it (and out of immediate reach of the jetwash). My first thought as the engines spooled up was "this is going to be <i>loud</i>" ... but it <i>wasn't</i>. Rather than the roar you'd hear when you were <i>alongside</i> the plane, all I heard was a loud spooling turbine whine ... until the jetwash roar itself returned to me echoed off mountains a few kilometers distant.<p>TL;DR: Jet engines don't make (much) noise, their exhaust does, and it has a markedly different velocity vector than the plane itself, or its engines, accounting for a different doppler signature.