What was suspicious about Julia's room in "The Speckled Band?"

Helen Stoner seeks Holmes' help because she fears for her life as she is convinced that she will die just like how her sister died. Helen and her sister Julia live with their stepfather Dr.Roylott. Two years ago Julia got engaged and just a fortnight before her wedding she died a terrible death under mysterious circumstances screaming, 'Oh, my God! Helen! It was the band! The speckled band!' She had informed Helen earlier that she often heard a 'low clear whistle' at three in the morning.

Now, Helen is in a similar situation. She has been engaged to one Percy Armitage and is to be married in the coming Spring season. But two days ago some repairs were started in her room and she has been compelled to stay in the very same room in which her sister died. Last night when she was sleeping on the same bed in which her sister met her tragic end,to her horror, she heard the same low whistle which signalled her sister's tragic death. So, in sheer fright  she has taken the first train to London and rushed to meet Sherlock Holmes to seek his advice as to how to save her life from impending death.

Sherlock Holmes and Dr. Watson visit Miss Helen Stoner at her step father's country manor at Stoke Moran. Holmes makes a careful and detailed study of the room in which Helen's sister Julia died, and where she has been asked to sleep now because some repairs are being carried out in her own room. Holmes' suspicions have been aroused because on examining the room from the outside he discovered that no repairs were necessary to Helen's room and that the repairs were only a pretext to make Helen sleep in the same room in which her sister Julia died:

"Pending the alterations, as I [Sherlock Holmes] understand. By the way, there does not seem to be any very pressing need for repairs at that end wall."

"There were none. I [Miss Helen] believe that it was an excuse to move me from my room."

"Ah! that is suggestive."

Holmes then examines  Helen's room from inside and discovers that there is dummy bell cord hanging just above her bed and a ventilator which opens into the adjacent room which is Dr. Roylott's:

"Very strange!" muttered Holmes, pulling at the rope. "There are one or two very singular points about this room. For example, what a fool a builder must be to open a ventilator into another room, when, with the same trouble, he might have communicated with the outside air!"

"That is also quite modern," said the lady.

"Done about the same time as the bell-rope?" remarked Holmes.

"Yes, there were several little changes carried out about that time."

"They seem to have been of a most interesting character -- dummy bell-ropes, and ventilators which do not ventilate.

At the end of the story Holmes explains to us that the dummy bell rope and the ventilator were necessary for the poisonous snake to slither down and bite its victim who would be sleeping on the bed down below:

My attention was speedily drawn, as I have already remarked to you, to thisventilator, and to the bell-rope which hung down to the bed. The discovery that this was a dummy, and that the bed was clamped to the floor, instantly gave rise to the suspicion that the rope was there as a bridge for something passing through the hole and coming to the bed. The idea of a snake instantly occurred to me, and when I coupled it with my knowledge that the doctor was furnished with a supply of creatures from India, I felt that I was probably on the right track.