I completely agree that the sound is incorrect,
but the sound on the ground and High speed in the air IS NOT the same,
it is just the opposite as it should be, see my explanation below.
The two files responsible for the annoying, far too loud, rumble noise are these:
A10InHeAmbL.wav
A10InHeAmbR.wav
Found in \Sounds\Effects\Aircrafts\Engines.
I just reduced the volume of these with a wave editor, so that's fixed.
But the biggest problem is the turbine sound wich INEXPLICABLY gets QUIETER with more throttle / speed, instead of LOUDER as it should be, and as it is in all other DCSaircrafts.