What causes the car to make a humming sound.

Bad or loose motor mount , If your exhaust is touching the body that may cause an odd sound. There's so many different parts that are spinning and could hum, can you narrow down a location. Is the hum at a constant speed/sound ? Is the noise different at an idle (less/more) ? Does ithe sound change with varied speeds or while turning ? Any other information would be nice.

Bad pulley could also hum.