He assumed that the rats were making the sound because he thought the sound was a familiar one. The doctor kept hearing the thuds from the roof. He heard the sound 4 times and only in the fourth time when the sound stopped a Cobra snake cam down.
Hari Shakthi Vel answered this
The sound was done by the snake.
Mayank answered this
He was not sure for the sound becauseit he was familiarwith that sound which wss of rats.literature is all about imagination my friend