OK, quick, easy diagnostic.

Remove the cassette, and/or install the wheel bypassing the chain, close the QR and spin the wheel. Do you hear it?

If so, it's the hub bearings (or related).
If not, put your finger against the freehub body and stop it while the wheel keeps spinning. Hear it now? If so, it's inside the freehub itself.

You can remove it, do a solvent soak and flush, set it on end to drain, then relube with a decent, sticky oil (not grease). Mount it, mount the axle, grease and adjusting the bearings and see if it's gone.

Or, once you've isolated it to the freehub, simply ignore it until here's a problem.