Need help - Road Train sound mod

[FishRSA]

All of the sound files are recorded from public online sources(not my own), edited and rendered by me with Audacity and exported as 16-bit .WAV files. 

 

The Infernus sound mod works 100% fine (I have both sounds in Bank_031 and all three sounds in Bank_032 modified and all of them work properly). However, the sound I just made for my Road Train (both files in Bank_077 an all three files in Bank_078 modified) doesn't work properly. Despite being the same 16-bit wave files, recorded and rendered in the exact same way, sound_002 of Bank_077 (this is the engine idle sound, which plays when the car is not moving and also when the car is moving without the W key being pressed) seems broken. 

 

(yes, i know it sucks, but i'm not finished yet give me a chance XD)

 

As seen in the video, it's as if the pitch of the sound is too high or something and it fails to play above a certain speed, then starts to play as the speed gets very low. I've included the Infernus mod as an example of what should be happening, and to show that it's (likely) not the fact that it's modded that's breaking it.

What could be causing this? I thought it was because the sound was too short, so I made it the same length as the Infernus idle sound, but it made no difference. I then made it even longer, but still nothing changed.