You need to determine what is buzzing, the fan motor or the compressor. Switch to A/C mode and see if the fan turns and the compressor runs.
With the unit off, try to spin the fan blade and see if it rotates freely and makes several rotations with a good spin. If it's stiff, you may be able to work it free, put some light oil in the bearings and/or tap the shaft with a hammer and see if it runs under power now.
