I wanted to add my story because until the end, my situation was the same. But it ended up differently. Everything from the initial humming and a slight caustic smell coming from the top left corner of my 2010 iMac made it seem like the power source was failing. When my computer wouldn't turn on, the LEDs confirmed the power source was not working. Because I haven't worked on computers before, I took it to Apple. Even though iFixit's videos on YouTube were great, I was not comfortable doing it myself. When the Apple tech opened up my computer, they said my computer was really dusty. I mean, the vent slot is pretty wide and probably lets in a lot of dust. The dust was preventing my computer from booting up, not a failed power source. From now on, I'll use the dust cover on my iMac when I'm not using it. I should have been doing this from the beginning. Whoops.