Step one is to isolate the issue to either the wheel bearings, or the freehub ratchet mechanism. That's easy enough to do by simply spinning the wheel in your hand holding the axle and feeling for snags or torque, then putting a finger against the cassette t hold it and seeing if it's dragging against the free-spinning wheel.

While you have it it's also easy to check the axle, by turning the axle in the stationary wheel and looking for oscillation at either ends.

Once you've (I expect) localized the problem to the freehub, then you can continue to try and flush it clear, but otherwise it'll need replacement.