With the wheel correctly secured in the bike, hold the rim and gently move it sideways. A knocking sensation may indicate bearing play. Cup-and-cone hubs can be adjusted, while wear in cartridge bearings usually cannot be corrected in the same way. Identify the hub type first.
DT Swiss recommends annual servicing for its hubs and says their industrial ball bearings can be replaced when necessary. Use the specific model’s documentation to identify bearings and tools. Suitable grease and the correct tools help avoid damage during servicing.
Note the hub model and the symptom: whether play is present with the wheel secured and whether rotation feels uneven. These details give a workshop a useful starting point for inspection.