Place your sound files in the following folder inside your mod:

<YourModName>/sounds/

Drop these sound files into your sounds folder.

Idle.wav
Run3.ogg
RunLoop.ogg
StartUp.wav

Final structure example:

<YourModName>/
│  modDesc.xml
│  vehicle.xml
│
└── sounds/
      Idle.wav
      Run3.ogg
      RunLoop.ogg
      StartUp.wav


Paste this inside your Harley XML located in sounds folder (usually named vehicle.xml, bike.xml, streetglide.xml, etc.).
Place it right under the <motor> section or anywhere before the closing </vehicle> tag.

<sounds>
    <motor>
        <start file="sounds/StartUp.wav" volume="1.0" pitch="1.0"/>
        <idle file="sounds/Idle.wav" volume="1.0" pitch="1.0"/>
        <low file="sounds/Run3.ogg" volume="1.0" pitch="1.0"/>
        <high file="sounds/RunLoop.ogg" volume="1.0" pitch="1.0"/>
    </motor>
</sounds>