It is always caused, or triggered, by the brake pads, but it doesn't mean the brake pads are worn out.

When the pad vibrates under braking, it produces a squeal. There are anti-rattle clips on the caliper that have tension to hold the pads in place, and there should be high temperature grease applied to the ears of the brake pads (where they touch the caliper) to eliminate the metal to metal contact that provides a path for the vibration, thus producing the squeal.

If the pads aren't worn out, it can be worth cleaning out the dust, and applying the grease, to solve a squealing problem.

There are other pads that produce less dust. I have used Axxis Deluxe, from www.zeckhausenracing.com with great results, as have many others here. Very similar pedal feel and initial bite, with no visible dust. Dave at Zeckhausen is great to deal with.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue