PSU failure or something else?

Hi, first of all excuse me if I did not post on the right section, please move it if there is a more suitable one.

My system specs are:
- Asus Maximus VI Formula
- Corsair AX 1200
- Corsair Vengeance 4x4 (16gb)
-Msi gtx 970
-Noctua u14s
-2 SSD // 3 HDD
-Windows 7

About 1-2 months ago I was hearing a buzzling sound and whole pc case was vibrating really like a mobile phone.
I thought it was the HDD cage so I replaced my Corsair 750D with a Fractal Design Define R5.
Since then pc boots normally but with no "beep sound" and I think this happened in the past but did not pay too much attention to it.

The more weird thing is that when I shut down pc it stops giving signal to monitor but pc continues working, fans psu motherboard everything working like it never closed. This happens about 2 out of 10 shutdowns not always.

One more thing is that HDD's were dissapearring from My Computer sometimes ive yet to see it for 1 day though so this may be fixed can't tell for sure.

2 days ago when I came back from work and pc was turned off, it tried to boot for 3 seconds shutdown and retried the same endlessly. I removed 3 dimms of ram and HDD's and booted normally. then tried again nothing, This was fixed as I figured out 2 sticks of ram were back to 1333mhz while the other 2 stayed in 1600mhz where they always were. I did apply 1600 for the other 2 dimms of ram and worked, but 2 out of 4 dimms of ram downgrading on their own is not normal at all.

The "no beep" sound as well as not shutting down normally sometimes continued so I did a reset bios settings (bios using the latest filmware 1603 // 8-15-2014) the buzzling sound was back again from psu most likely.I should note I just put cpu fan to silent 400rpm and rams in their 1600mhz again later.

In a system that needs like 500watt max a 1200 psu should not have fan working at all right? its the only thing I can hear out of whole pc.

Since too many things happened I can not exclude something but everything seems like pointing out psu in my belief at least, I dont want to blindly change it to find something else is wrong just like I did with my pc case, so I would like to confirm it somehow..

If I think of it way back I remember a noise on my silverstone tj07 (the case I was using before 750D) with fully custom watercooling but I had the imagination it was the d5 pump so I got rid of watercooling back then.

The pitching sound (not as audible as in the 2 previous cases after 5 days of using the Fractal R5 noise dampening case) is still here just restricted, so is the shutdown problem.

Give me some advice please, could it be PSU? Motherboard? RAM? Anything at all?
Im more worried that if something dies on me, can it take other components with it as well?