PC won't turn on and the NIC LED flickers for 50 seconds?

Question

So i have had this issue for a while now(mostly after the last cleanup perhaps), but previously it did turn on after just sitting behind the power cord for 5 minutes or so, but now it won't even turn on after 20+ minutes etc...

And when it has sat without any power for a while, the NIC LED starts to flicker(after given the first power again) with identical noise coming out from the Power-supply - and when the PSU noise stops, the LED will also turn itself fully on and stops flickering... So what could be the issue here?

I don't see any damaged capacitors on the motherboard because they are all just smooth and flat from the top, not unlike some old ones that had those 4 triangles on top... I might also try to listen for the noise from the open side of the PC, but i am pretty sure that it only comes out from the behind, from the PSU...

(i also tried to bypass the power switch by connecting the 2 pins together and also replaced the cr2032 battery(old one was like 1.35-1.7V?), but it still won't turn itself on...)

Answer 1

Well, it seems like when any Leds are flickering on the motherboard and the PSU makes the same exact noise(in sync), then it's probably the PSU's fault, because a replacement motherboard did the same exact thing until i plugged in a new Power-supply... And the weirdest thing is that somekind of square/digital PSU tester showed that all voltages are fine on every output and it even turned on an old HDD with the green+black wire jumper trick...