It's OnyxPL's
Audi 80 
specifically, the Quattro variant.
I had a look at the .soundscript file, found it uses the following .wav files:
- audi_idle_quattro.wav
- audi_2000_quattro.wav
- audi_4000_quattro.wav
- audi_5000_quattro.wav
- audi_6000_quattro.wav
They are all mono files.
I replaced the original idle wav with a "louder" one, but it sounds just the same, in game.
I was also wondering about the "gain_factors" in the soundscript file. After some googling, I'm guessing this is responsible for the pitch of the sound, not the loudness.
Any other suggestions?
