The brief
The X4 had developed grinding and whining noises when the brake pedal went down, the kind of sound that means the friction material is running out. He brought it in before the braking performance dropped any further, which on a heavy SUV is not something to leave. Grinding when braking usually means the pads are worn down past their friction material, so the backing plate is starting to touch the disc, and a disc that's been worn unevenly also whines and judders. Brakes are a wear item, and there's no nursing them past the indicator, so when they get to that point the pads and the discs go together, because fitting fresh pads onto a lipped disc just beds them into the same uneven surface.
The diagnosis
Wheels off, the front pads were worn down to the wear indicator and the discs had a clear lip around the edge and measured past the minimum thickness on the gauge, so they were done too. Fitting new pads onto worn discs doesn't make sense, so it was a set job, front pads and discs together, to reset the braking properly.
The work
The front callipers came off, the worn discs were swapped for new genuine BMW-spec rotors, and a fresh set of pads went in. The slider pins were cleaned and greased so the callipers float freely, everything reassembled to torque, and the brakes bedded in on a controlled road test, a series of measured stops to lay an even layer of pad material onto the new discs. The road test confirmed the grinding and the whine were gone, the pedal was firm, and the bite was back.
The outcome
No grinding, no whine, a firm pedal, and full, even bite back at the front. The X4 went home with the braking reset to like-new. Brakes are a wear item and there's no nursing them past the indicator, so doing the pads and discs together gave the car back proper, confident stopping power.