If the brakes are squealing, there's a good chance it is the brake pad wear sensor. A new design was implemented in January 2006 and can be identified by its green color.Google+
Car Problem Reports
BMW 525xi New Brake Pad Wear Sensor Design Will Prevent Squealing
BMW 525xi Problem
Model Year Affected: 2006
Visitor, 2006 BMW 525xi, 85,000 mi
The brakes consistently squeaks. When I took it into the dealership, I was not due for any brake work, but they could not explain why the brakes squeeked.