My female cat, Cleo, makes that sound all the time. When I talk to her, look at her, when she's walking around the house, when she's bored...she usually prefaces a real meow with that sound too. My male, Alex, never makes that sound at all. I was wondering why myself.
My Beloved Corkie used to answer me like that. If she was asking me a question or saying "REALLY!" the Brrrr would end on a high note, but if she was agreeing with me or NOT asking a question it would just be sort of flat and if she would be saying something like she was disapointed it would end on a down note!