What two sounds did Helen hear after she was awakened by her sister’s screams? What did her sister then say?

In "The Adventure of the Speckled Band," Helen Stoner (the protagonist of this story) approaches Sherlock Holmes with a mystery regarding her twin sister's death. She begins by offering Holmes background information about her family, their childhood travels, and their current living arrangements. During this conversation, she shares that their family grew up in India. Their mother, a widow, ended up marrying a man named Dr. Roylott, who was known to commit acts of violence. When they married, the twins' mother brought a large amount of money into their marriage. Later, the family moved back to London, England. The girls' mother, after arriving in their home country, died. In her will, she left the majority of her money to her children upon their marriages. (Women, in this time, were not generally willed money. It usually went to the males in a family. Therefore, they could not receive the money until they married.)
This left the girls, while unmarried, to live with Dr. Roylott, their stepfather, in his old family mansion with his strange and dangerous pets (including a cheetah and a baboon). Not surprisingly, the girls did not feel very safe while living in their stepfather's home. It's also important to recognize that their stepfather didn't want the girls to socialize in their new community. Perhaps he didn't want them to meet any men that they might want to marry.
Even so, Julia Stoner did end up meeting a young man who wanted to marry her. Her uncle didn't seem overly troubled about their engagement, but soon afterward, she started hearing strange noises in the middle of the night. The wedding date grew nearer. One night, Helen, her twin sister, heard Julia's scream and ran down the hall to her room. She explains to Sherlock:

"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."

Her sister looked like she is in great pain, she was pale with a look of fright, and she barely got out a few words before she died. Her words were,

"Oh, my God! Helen! It was the band! The speckled band!"

Julia explains that it looked like she had more she wanted to say, but the pain took over too quickly and she died before she could continue.
Julia's death leaves Helen frightened. She imagines it may have had something to do with her stepfather, but no one can figure out how Julia died. When Helen is talking to Sherlock Holmes, she has recently gotten engaged. She is worried that she might also be in danger and is asking Sherlock Holmes to get involved in solving the mystery of her sister's death (and hopefully keeping Helen safe from similar harm).
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