I use an Asus Xonar DS soundcard, maybe there's some compatibility issue there.
In sound settings there's a "Sound Sources" setting. Normally I keep this set to 128, but I've found that after setting this to 16 the idle vehicle sounds will go away after a couple of minutes. I'm guessing this is because you limit the amount of sounds that can be played to 16, so once 16 new sounds has been played after the idle vehicle sound starts playing it will replace the vehicle sound because it's out of channels to use.