depuis and Imparfait

Bonjour abt,
If I am correctly understanding your question: «Comme je ne faisais pas de bruit depuis un moment, maman est venue voir ce qui se passait.» ---> As I was quiet for a while, mother came to see what was going on. Because you indicated that this came from a book, it appears to me that the author was writing descriptive text. Also, the quietness had been going on when the mother came to see what was happening, it had begun prior to her entering the area and was continuing up until she entered. In French this is a normal use of l'imparfait and depuis.
« the action started in the past, but is still ongoing at the time we speak.
In French, you use depuis to express an ongoing duration.»
«Note that L'Imparfait is also used for descriptions, i.e. to help visualise the setting or atmosphere in which an action took place, characters, location, time frame ...
In a similar way, L'Imparfait is also used to express opinions about the past:»

J'espère que ma réponse vous aiderait.
Bonne chance et bonne continuation dans vos études en français, la langue de Molière et qui a été utilisé par le monde français depuis l’époque d’Hugues Capet

Merci Ron! Yes, your explanation makes sense. I was just not expecting the 2 to go together.