Pip might never have encountered the convict if he hadn't started to cry.

...and that the small bundle of shivers growing afraid of it all and beginning to cry, was Pip.

Magwitch has been hiding there among the gravestones. He is undoubtedly well aware of Pip's presence but would most likely prefer to remain hidden rather than take the risk of revealing himself to the boy. Naturally he would expect a boy to go running home to tell his parents what he had seen. Magwitch might even be thinking that he might have to kill Pip if the boy should happen to see him there. How else could he keep Pip from telling others where he had seen him? Pip might even let out a scream when he saw the terrible man materialize among the gravestones--so by "noise" Magwitch not only means the sound of Pip's crying but the much greater noise Pip would make if he were to let out a scream of terror or a cry for help. 

When Magwitch says, "Hold your noise!" it shows that what he is mainly concerned about is noise which would attract the men who are searching for him. Magwitch is forced to reveal himself when Pip starts crying. This changes the situation radically. Now Magwitch either has to kill Pip or else find some means of ensuring that the boy will not tell anyone he has seen him. When Magwitch says:

“Keep still, you little devil, or I'll cut your throat!”

he really means it. Pip's life is in serious danger. But it occurs to Magwitch that Pip can be a great asset rather than a threat. This is especially the case when Magwitch learns that Pip lives with a blacksmith.

“Now lookee here,” he said, “the question being whether you're to be let to live. You know what a file is?”

“Yes, sir.”

“And you know what wittles is?”

“Yes, sir.”

Magwitch sees that Pip can easily be frightened into doing anything he tells him to. Pip brings the convict food, brandy, and a file the next morning because he is terrified of what might happen to him if he breaks his word. However, Magwitch takes these things as an act of kindness, and the rest of the novel evolves because of the hunted man's gratitude.

"I tell it, fur you to know as that there hunted dunghill dog wot you kep life in, got his head so high that he could make a gentleman—and, Pip, you're him!”

The words of a character are often a great tool in the technique of characterization.  Certainly, in nineteenth century England, one's diction and dialect placed a person into a certain social class.  With the convict's harsh invective against the crying orphan, Pip, there is the indication that this coarse man has lived a life devoid of sympathies himself, for he knows not how to display any to others. He is 

A fearful man, all in coarse gray,...who limped and shivered, and glared and growled, and whose teeth chattered in his head as he seized me by the chin.

Further, he calls Pip a "young dog" and turns him over and shakes him.  Instructing the boy to bring a file and "wittles," the convict cruelly threatens Pip with a young man who has a secret method of cutting a boy's heart and liver.

The passages from Chapter I of Stage I of Great Expectations that illustrate character also exemplify the didacticism of Charles Dickens that is often extravagant at times. For, here in this opening chapter of pathetic fallacy, the ambiguous and cold,gray mists of the marshes are in harmony with Pip's heart; on the other hand, the ill-treated Magwitch, the convict, whose childhood was spent as a gamin of the streets, a child who wanted any kindness, has no such sympathy for Pip because he himself has never been shown any. Instead, he has had to "hold the noise" of crying as it was an act of futility in his childhood. 

This quote comes from the opening chapter of this book, and with these words the character of Magwitch is introduced into the novel. The sudden introduction of his voice, which is the first indication that the reader and the youthful Pip receives of his presence, makes his appearance both a surprise and somewhat terrifying:

"Hold your noise!" cried a terrible voice, as a man started up from among the graves at the side of the church porch. "Keep still, you little devil, or I'll cut your throat!"

In terms of the opening phrase, to modern readers the use of the word "Hold" in order to indicate a ceasing of noise is slightly peculiar, because that word is not used in this way in today's world. This phrase therefore is an example of dialect, as it reflects both the way that Magwitch spoke himself but also the way English was used in the time of Dickens. The use of the verb "Hold" almost presents an implied metaphor as the noise is imagined to be something that can be held and covered in Pip's hands, and therefore silenced. This phrase therefore captures the way that English has changed since this time in history, and is therefore an example of dialect.