The most common and most likely answer to this crossword clue is the 6 letter word SHRILL
I've seen this crossword clue in Financial Times
S
H
R
I
L
L
More clues leading to the result SHRILL
- Keen husband dipped into small brook
- Keen to immerse husband in southern brook
- Small hotel “run poorly” – sounds harsh!
- Quiet small stream not at all quiet
- Quiet running water becoming piercingly loud
- Keen for mum to run off
- Quiet little brook anything but quiet
- Word of honour with which respected Indian is addressed by learners being keen
- Noise that might disturb quiet stream
- Excessively strident US con man’s side kick trapping shopper ultimately
Buddy explains!

The straight part
We know that "not at all quiet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not at all quiet = shrill.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 SHRILL mean?
SHRILL noun- A shrill sound.
- To make a shrill noise.
- High-pitched and piercing.
- Having a shrill voice.
- Sharp or keen to the senses.
- (especially of a complaint or demand) Fierce, loud, strident.