1."a brief strange noise" what is the grammatical name given to this expression?

2. What is its function.?

What is the function of the expression a brief strange noice

It is noun phrase

Noun phrase

Noun phrase is the phrase that does the work of the noun

This is a noun phrase. It includes a noun (noise) and all its modifiers (a, brief, strange).

http://www.chompchomp.com/terms/noun.htm
Scroll down to "Understand the functions that nouns provide in sentences" and read about nouns, including noun phrases.

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It is a noun phrase because it is a group of words made up of an article (a, an, and) an adjective and a noun. Which is used usually or often as the subject, object or even as a noun.

Noun phrase

It is a noun phrase 🤗🤨

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A subject complimen.t