What two sounds does Helen Stoner hear on the night of her sister Julia's death?

The two sounds that stand out in Helen's memory of the night of her sister's death are described by her to Sherlock Holmes at their initial meeting in the early morning.

As I opened my door I seemed to hear a low whistle, such as my sister described, and a few moments later a clanging sound, as if a mass of metal had fallen. 

The reader will not understand the meaning of either of these sounds until late into the story. The low whistle was used by the evil Dr. Roylott to recall his poisonous snake, "the speckled band," from the room next to his. He had trained the reptile to return in response to his whistle. He would blow the whistle at around three o'clock in the morning in order to make sure the girl would still be sound asleep and not discover a snake in her bed. But evidently he did not realize that Julia had just been bitten at the time he blew the whistle--and perhaps Julia herself did not realize it until the poison started to take its effect. She woke up and struck a match in time to see the snake slithering back up the dummy bell-rope. 
The "clanging sound" was made by the door of Dr. Roylott's steel safe, which was where the captive snake spent most of its time. Roylott must have heard the commotion outside in the corridor, but he obviously had to recapture the snake and lock it in his safe before he could put in an appearance. One of the things that arouses Sherlock Holmes' suspicions when he is examining Dr. Roylott's room is that steel safe.

“What's in here?” he asked, tapping the safe.


“My stepfather's business papers.”


“Oh! you have seen inside, then?”


“Only once, some years ago. I remember that it was full of papers.”


“There isn't a cat in it, for example?”


“No. What a strange idea!”


“Well, look at this!” He took up a small saucer of milk which stood on the top of it.

Roylott was using the milk as part of his training of the snake to return at the sound of the whistle. This would appear to be an early example of Pavlovian conditioning.