90 percent of the time. It is heard slowing down, at speed less than 10MPH as you are stopping. This type of noise is caused by harmonic vibrations on the back of the brake pad. There is a goop to apply on the back of the pad to stop this type of vibration, and I think BMW should do this just because it is a perception by me an everyone riding in my car that a BMW shouldn't sound like a 15 year old truck. Even my mother is shocked.