The most common and most likely answer to this crossword clue is the 8 letter word WHIRRING
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Buddy explains!

The straight part
We know that "vibratory noise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vibratory noise = whirring.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WHIRRING mean?
WHIRRING verb- To move or vibrate (something) with a buzzing sound.
- To make a sibilant buzzing or droning sound.
- To cause (something) to make such a sound.
- A whirr, the sounds of a whirr.