Mine did the same thing. Yours will get worse until it screeches every time you apply the brakes. BMW has a problem with the brake sensor/pad. Take it in and they will replace the sensor and pads stating that a new design is out shortly and they'll replace with something that doesn't make this noise once it comes out. i think this was a big problem on the 5 series or 7 series, and doesn;t affect many X5s, but alas, it does a few. I bet yours is a 2007 - mine is.