Mine had an annoying knocking on a rear right pad. A pad was loose. Check the long thin metal holding bracket. Ensure it's clipped correct to the caliper.
What type or brand of brake pads are on it? I've seen some that squeak due to compounds used to make them. I always go with ceramic pads. On the caliper side, open up the prongs so the pad fits tight.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)