The black thing with Everbilt name is a pump to dispose of condensate which drins from ac. The hum night be coming from transformer inside ac unit. If the black device is humming but fan on top isn't turning try moving fan blade with a toothpick or something. If blade can't be easily moved the pump is probably jammed.

Thanks @klem1 and @stax. Confirming that the condensate pump is in fact making the humming sound (unplugged the cord and the noise stopped). Also, its fan has been running - it just seemed like it isn't until I put a toothpick in.

Worked like a charm @stax! When I first put water in, the red Alarm light came on. It took a few mins for the pump to empty out the water but since then, the noise is gone.

Thanks so much!

Hi again,

It seems like there's something more going on. I ended up replacing the condensate pump last night, just to be sure. The a/c ran fine overnight, but now the indoor unit is running, the condensate pump has all 3 lights on, but the outdoor isn't running. It's hot in the house, and I'd imagine the a/c would be running constantly if everything was working fine.

Thanks again for your advice!