Actually, I've come across this sound issue too. And the sound can actually vary. For example, I played a mission in the C-101CC where I did a rocket attack. Then I played the mission again, and during takeoff, every time I hit a bump on the tarmac, it would make a rocket firing sound. Kinda funny. :) I'll try to reproduce.
EDIT: Ya, seems to be happening in both release and open beta branches. Here's the simple test I did.
1. Start the C-101EB Caucasus take-off quick mission.
2. Take off from the runway and then crash your plane and end the mission.
3. Restart the mission.
4. Take off from the runway again and you should hear some odd sound (seems to vary) while rolling down the runway. If you don't hear a sound, try repeating the steps.
Thanks.