Pulley bearings can be replaced without loosing Freon, accessed from the front of the compressor. You may need to remove the radiator to get enough room to work. But I would think a bad bearing would make noise all the time. And a belt usually make noise only under load, as when the compressor is engaged.

You say the noise stops when the compressor is engaged? Could be a problem with the compressor clutch. When the compressor is not engaged, the pulley and bearing turn, but the clutch plate mounted to the compressor shaft doesn't and just misses the pulley by a small gap. When the compressor is engaged, this plate is sucked against the pulley and turns with the pulley. It is possible that this plate is jiggling and rattling against the pulley, and I think the plate is usually mounted in rubber. Maybe your oil spill lubricated the assembly up and caused the plate to slip in the rubber mount, possibly accelerating an existing problem. Or something broke loose in the plate assembly. The clutch too can be serviced from the front without loosing Freon.