+1 on Dabac's points. have a local shop remove the freewheel, or buy a tool and learn how to do it yourself, and soak it in solvent to flush it out.
The best way is in an old cookie tin or coffee can using naphtha or mineral spirits. Give it a long (hours or overnight) soak, agitating occasionally. Near the end when the solvent has loosened everything agitiate more aggressively to flush it clean.
Shake it dry then leave it in a warm place so the last of the solvent evaporates then run some oil in at the gap between the moving parts until it feels and sounds nice. Drain it on a paper towel overnight then install.
The wash and oiling can be done on the wheel, but it'll make a mess as the stuff spins out all over the place.
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