After buzzing noise iMac wont turn on

Hi all,

In November I updated my iMac with new and more RAM and a SSD. It worked fine till January. From that point the Mac began making buzzing noises. When I turn on the brightness the noise gets louder. So it wasn't the fan.

Now, 2 months there where no additionel problems, only the noise. Today I shut down the Mac and after 45 minutes I turned it back on. But the Mac didn't answer. No power, no noise, no screen. I unplugged the cabel, but still no power…

What to do? Replacement of PSU?

Thanks!