When you tear down next, look for missing board components or any that lean at an angle they’re not supposed to. Canned air has been known to shear poorly-attached caps or whathaveyou clean off the board. Or maybe one of the wires in the 12v cable is broken and it’s delivering power just to most pins… The paperclip test just tells you if the PSU is dead dead, are you able to test with a multimeter instead?
Buzzing while the power supply is off makes me wonder if there’s literal bugs in it, if it’s buzzing while it’s unplugged. I suppose the buzzing could also be remaining power discharging, if it happens just after it’s been unplugged.
NH-D15 is default for me. Incredible performance, whisper-quiet. And if you need new brackets for an upgrade, the company will supply them free of charge!