During hard braking, possibly the steering axle. The Z1 has a long steering axle, which only is guided at both ends, at the steering wheel and at the steering casing.

Before entering the engine compartment, the two parts of the axle are connected. This connection exists of an axle moving into another. This is to compensate axial movements, to adjust the personal setting of the wheel or to avoid the wheel ending up near your backbone in case of a frontal collision.

This connection can be adjusted by a nut that can be tightened with a spanner. If this clearance is to much, you can check this by trying to move the axle up and down, it can cause an irritating rattling noise.