Unreliable and humming iMac fixed
30 minutos - 1 hora
My iMac started acting up after someone yanked the power cord while it was still running. After that day, it started humming whenever connected to power and sometimes would go days without turning on. I could hear intermittent humming while trying to turn it on but it wouldn't do anything until the humming went steady.
The repair was fast, about half an hour. Just follow the iFixIt guide and you'll be ok...
It's good to have an extra pair of hands, whether to hold the display while you (dis)connect every micro cable attached to it or to hold the flashlight while you look for that fallen screw...