If other aircraft are setting behind you on the runway, you are hearing the ai sound file for that/those aircraft running. Each aircraft should be sound mapped to either a generic ai sound file or a specific ai sound file. For instance the sound.xml will have a generic ai sound tag for any fighter that does not have a specific ai sound tag and corresponding .wav file. The tag in blue is the generic tag and is the game default sound for any fighter. My P-51d as you can see has a specific tag (in green) and the game will use that one instead of the default. Two ways to reduce the volume of the ai sound file is either to reduce the volume of the ai.wav file or include this Atten="0" in the sound tag you wish to reduce. You need to adjust the number in red to change volume level. I believe 12 is lowest and 0 is highest but I don't recall exactly. Someone here can refresh my memory. Remember If you adjust the default tag it will lower sound for all aircraft that use that tag.
<aircraft>
<sound file="ai_fighter.wav" />
</aircraft>
<p51-d>
<sound file="ai_p51d.wav" />
</p51-d>