OK, so we know it's freewheel/cassette related.
Odds are it's dirt or damage within the freehub, which you might be able to oil.
But before you go there cross your fingers and remove the cassette. If you're lucky, you'll fins some crud, usually string or similar, wrapped around the hub behind the cassette.
Then, you can keep your fingers crossed as you oil the freehub body. Some (not enough IMO) have an oil port under the cassette, but odds are that you'll need to work the oil in past the end seal.
If that doesn't work, remove the freehub from the shell (there are tutorials on the net), soak it in mineral spirits, drain it then oil it. If that doesn't solve anything replace the freehub.
BTW- you don't need to remove freehubs to clean or oil them, but it can be messy if done on the wheel.
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