Need help deciding if there is a PSU failure

There is a problem I have been trying to solve for quite some time already. My initial configuration was:

Processor: AMD FX - 8350
Motherboard: MSI 990FXA-GD80
Power Supply: Corsair RM750
Memory: Kingston HyperX Fury 16Gb (2x8Gb)
GPU: PowerColor PCS+ R9 290

As soon as I assembled this configuration I did not use it for almost a year (it was initially bought for one project that in the end I did not participate in). But it was plugged into the wall along with my Laptop + Display. I did tu it on from time to time for small tasks. One day I woke up from noises comming from the PSU (the type of noise you can sometimes hear when plugging something to the wall, the "sound of electricity"). So i unplugged the PSU cable from the wall and decided to take a look at the problem in the evening. The computer tued on normally and no sounds were comming from both the PSU and the computer itself. I decided to keep it unplugged just in case. The next week I have tried to tu the computer on and nothing seemed to show up on the screen. All lights, the CPU fan, GPU fans, case fans were working but nothing was showing on the screen and the motherboard was showing an FF code.

I did a research on the web and found a manual how to debug the computer problems on SuperUser. In the end after unplugging everything but the CPU and CPU fan the motherboard was still showing an FF code so I have decided that it is either a PSU, motherboard or CPU failure. I did test the PSU with multimeter myself. According to this test the PSU was in perfect shape and since CPU's failure rate is generally way lower than with motherboards I took the motherboard to the shop where I bought it for a replacement. And this is where the funny thing happened. Somehow my motherboard was lost and I got a full refund on my purchase.

So I bought an MSI 990FXA Gaming motherboard. Assembled my computer and the same issue was still present (nothing on the display while everything is spinning). I took PSU to an authorized Corsair service in my city yesterday and they did test it with their own PSU Tester and also plugged it into another computer they had. They told me that that it is hardly a PSU failure.

So I found a person who had an AMD FX-4300 and decided to use it for my own test. So i replaced the CPU and the system finally booted. Which leads to a conclusion that it is probably a CPU failure.

But what I have found to be strange is that when I plugged in the PSU into the wall and tued it on with the switch on its back I heard the same sound that I heard back then comming from the PSU and the CPU fan started spinning even though I did not tu on the computer itself.

And this is the end of my story at the moment. And I am absolutely confused and do not know what should I do next? What do you think is the weak link in this situation? One thing to note that there is no PSU more powerful than 500 Watt at the computer services in my city so I will have to buy one.

Thank you for taking your time to read my story and I would also like to thank you in advance for any thoughts on my situation.

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