Having read "EVGA Hadron Air Really loud PSU fan, even when in desktop with very little load" and various comments /solutions I thought I'd create a separate thread describing how I dealt with the problem.
 
The pitch of the the PSU fan when it gets up to speed is awful. I dont think its faulty, I just think that's the way the fan is. The fan starts off dead quiet but the lack of airflow at the bottom of the case (due to the size of it) means the PSU gets hot and ramps up the fan control. As mentioned in the other thread - one solution is simply to dial up the top exhaust fans even at low load to create more airflow (it works but the pitch of the PSU fan noise was still unpleasant at higher loads).
 
Having opened up the PSU and determined replacing it would be fairly straight forward I set out to find a quiet replacement. Trouble is the fan is 40x40x28mm (or 30mm deep, not sure as it seems to be somewhere in between..) and you cant really seem to find silent fans of that size as its only server fans of that size, not consumer (correct me if I'm wrong).
-My first solution was to try a Noctua NF-A4X10 FLX 40x40x10mm fan thinking "if I get an expensive fan, it can just move more air and be smaller". I was wrong. The fan wasn't moving any way near enough air and as a result was spinning at full speed all the time =  no real gain there :/ (to be fair that was actually just as loud as the stock PSU fan on low...)
 
-So I then bought a Scythe Mini Kaze Ultra 40x40x20mm silent fan hoping that would be enough and just as I was fitting it I started to think "what if I stack them up?" 20+10 = 30 mm right? Anyway I did that (picture below) and touch wood, it has shut my PSU up for good (even at full speed both those fans together are still quieter than the stock one in mid RPMs, and the pitch is oh so much more bearable)
 

 
This is not a cheap solution but its done the job and Im now eyeing up a 1TB SSD to get rid of the HDD spindle noise.. 
 
FYI spec is:
Core i7 4770k @ 4.3GHz
8GB corsair XMS 3 (soon to be 16) @1800MHz
Gigabyte Z87n
Gigabyte Radeon HD 7750 (I'm not a gamer, well not yet...)
2x 1TB HDD
2x 120GB SSD
 
Fans (it is dead quiet when idle, and very bearable on full speed):
2x Compucase CF-V12H Cougar (orange looks sick through the top grill ) (2x £13)
Noctua NF-A4X10 FLX + Scythe Mini Kaze Ultra  in PSU (£11 + £9)
So fairly hefty spend on fans but can you put a price on silence? well you can... 
 
Measuring sound level using an app on my phone placed in front of the case:
Baseline with computer off: 28dB
Idle with stock everything= 43dB min, max = 52db
(need to measure it again since I replaced CPU fan but it was 44db on max load and 35 idle, im guessing its around the 32 mark now idle)
 
Anyway hope this helps someone, maybe if you were hesitating as to whether you wanted to try replacing fans. 

Thats awesome Jfeil! Wish I'd thought of that now... definitely going to try it

still happy with it :). The scythe is definitely louder than the noctua (to be expected really, its bigger and cheaper) so I ended up connecting the +/- power of that to the mobo's connector for the case fans, and leaving the noctua connected to the PSU fan control. I think the PSU fan control is a bit too aggressive to be honest and that was half the problem with the stock fan.
 
As Jfeil suggested I've now lifted the case up and that seems to have helped a bit too
 
After all my tinkering I was tempted to flip the PSU over and take the cover off and then just let the Graphics card fan cool it, but I have no idea what the implications of that are (other than dont stick your hand in the case whilst its plugged in..) so I've left it for now.

sorry I really should have mentioned that in the original post. I was going to solder it on but the decided to use the OmniJoin adapters that came with the Noctua fan.
What I did was cut the current fan wire about half way down, strip the black outer plastic and then connect it as described in the Noctua manual.
 
For the other one I just ran the wire out of the fan cut out in the psu over the GFX card and then connected it to the Case Fan header on the mobo (as well as the case fans obviously..)

Yeah probably should have mentioned the warranty thing..
Glad it worked you for you schiffhauera! Out of interest what CPU fan are you using?

Have you got that mounted at the top instead of the 2x 120mm fans?

Thanks! I think I've got the mental picture right :)

And how about the  Scythe fan and noctua do you have finished picture? I'm just curious because I'm planning to build in this case . Now I'm concerning about that psu PROBLEM.
 

thanks :)

Great thank you, I did exactly the same first buy noctua and tryed it as a single fan. But not much success and after reading your post get the scythe together with noctua. The result you know.

Hi there is no probe to monitor the PS unit. But I run setup with 40x40 Nuctua plus scythe fans and the temps is ok. I have Intel core i74770K and GTX770 watercooled during gaming (Arma3 at ultras) my psu is very warm but not hot so I can touch it, CPU temp is around 50-60 C, GPU is 50-55 C. Sure I think that the unit works on its highs and I plan to put additional exhaust fan in the case. (I have two intake 120mm fans over the EK radiator so the hot air is inside the case, now I think how to blow it away)

Hi, yolof
It is very easy. First of all you need to unscrew 2 screws on the back side of the case and two more on the bottom which are fixing the PSU. Then you need to remove the PSU from the case and remove all the screws from it. That`s it. Next you should follow cguesty post on top of this thread.