Chosen Solution
My 27” iMac Mid 2011 power supply makes a constant electrical clicking/ticking noise when the machine is on standby ie. power cable in, but imac not powered on. I have replaced the PSU with another pre-owned one, which basically does the same thing. Is this normal or have I got two dodgy power supply’s?
It does sound like a difficult problem to isolate after trying a second power supply. A clicking sound could come from a few places. You’ll need to start at the power source your AC outlet first. An outlet that is old or loose could be loosing electrical contact with the cord. So as your room heats up and cools the connection breaks and reconnects. I would also make sure the outlet its self is properly wired as well using a outlet tester like this one GFCI Outlet Tester 48-250V You also might want to talk with your power provider to see if they can place a power meter at your house to monitor the power over a period of time if the ticking noise is not consistent. Replace your surge protector or better yet get a UPS. If the outlet looks good try using a different power cord. See if diagnostics picks up something restart your system press the D key to get in from a USB wired keyboard.
