Answer:
With a trembling hand, Agatha reached out and touched the knob. All at once there was a loud song call shriek.
Explanation:
We naturally expect the mood of the subject to be in a trembling state; which implies that Agatha was in fear. Also, "there was a loud" wouldn't be a complete thought without such details as "there was a loud call song shriek", which tells us the sudden and frightful nature of how events were unfolding for Agatha.
