Is my GTX 760 faulty, or my PSU? Help me diagnose.

First things first, my PC has been performing nicely since I built it in December 2013. Two days ago, though, I decided to upgrade from an i5 4570 to an i7 4770 and add an extra 2x 4gb corsair vengeance ddr3 ram (upgraded from 8gb to 16gb). However, as is my custom after installing new hardware the every first thing I did was stress test it (using memtest for the ram and intel processor diagnostic tool for the CPU). It passed all tests without a single error. That same evening I used my PC, satisfied everything was working fine, and played AC4 Black a Flag in 3D mode without a single lag, noise, crash, bsod, or any other issue. At the end of the day I turned my PC off, as I do. The next day I go to turn it on and shortly after startup everything is lagging like heck. PC stopped responding then unfroze itself a few seconds later, etc. so I thought maybe something was corrupted so I wiped the os and reinstalled afresh, then reinstalled all drivers afresh. I thought that had solved the issues but then the lagging started again. I did a windows experience test and heard, for the first time, a coil whine type squealing sound coming from the PSU. I started googling for possible solutions and as I was scrolling I heard this scratching/clicking sound while I was scrolling (which subsequently stopped when I stopped scrolling). As the coil whine sound only happened during the graphics part of the windows experience test I started to suspect the GPU may be faulty (initially, you see, I thought it was the GPU making the whining sounds...it was only this morning, when I took the cover off the PC, that is realised it was the PSU...which, FYI, is a Corsair CX750). I chatted to nVidia and their first response was, "have you got the latest drivers?" I hadn't, I had driver 320.49. So I proceeded to update the drivers. After it downloaded them, without issues, it started to install them. Screen went black and there was a windows loading circle cursor on the centre of the screen. "Don't worry, this is normal" nvidia said. But shortly after they said that I got a BSOD: http://tinypic.com/view.php?pic=5drltk&s=8#.UxHOaX8gGK0 (I had set my computer to record memory dumps and not to automatically restart after a BSOD). However, after a reboot there was no memory dump file anywhere. I know the location of said files, before you ask. I even did a wildcard search on *.dmp, no results found. I then decided to disable gtx 760 and plug in onboard graphics. Did a windows experience test....no coil whining sound whatsoever. Switched back to gtx760, re-ran experience index, coil whining sound again...but only during direct3d part of the test. At that moment I realised it was in fact the PSU whining, not the GPU. It was definitely coming from the PSU. So I thought maybe the cable might be dying so I switched to another cable...same problem. Eventually I took the gtx 760 out of my PC and the issues seemed to stop after that. As a result of this I have absolutely no idea whether it's the GPU causing this, or the PSU. But I'm pretty sure it's not the ram or the CPU judging by the fact I've literally just finished doing diagnostics on those and they pass every single one. And how come I was battering the hell out of my PC in 3d gaming two evenings ago and then this happening from last night onwards...with nothing having changed in that 24 hour period? I've spoken to friends, colleagues, nVidia support. Checked event logs, tried to get my PC to write dumps (I've set it up so it should), the lot...I cannot get my head round this. So I've come on here to see what you guys think.