Note: I think it only happens (or at least *starts* happening) when you're country at war. Therefore, to duplicate:
1. Play as Brazil in 1836.
2. Zoom in 2-3 levels past map view. That is, starting from paper map, zoom in 2-3 more times.
3. Unpause the game at speed 3 and wait a few in game weeks. Move your camera around Brazil, admiring the dozen or so carriages carrying supplies to both of Brazil's front lines.
4. At some point the carriage sound will continuously loop and become thunderously loud. Multiple sources will play the sound at the same time, causing a massive increase in volume.
The channel responsible for this audio is 'SFX'.