Ṣarrah suggests a loud commotion or the tumult of a group moving together. Here it condenses the sound and collective motion of the herd. Lam tuzayyal indicates that the group has not yet fully separated or scattered.
The line captures a precise instant: the herd is moving noisily, its leaders and stragglers can be distinguished, but the group still retains its collective form. Speed and cohesion coexist in the same scene.