RMA GS600w PSU fan runs at full-speed when idle

EDIT: Solved it, months later changing the chassis fan. It was the case fan that made that horrible noise.

Just an FYI, in case other people had similar issues.

First things first, system specs (as it's not my main, but an alt computer):

 

Mobo: Asus p8b75-m/csm (Brand new)

PSU: Corsair GS600w (RMA replacement, 'brand new' was sealed)

CPU: Intel i7 2600 (stock heatsink fan)

GFX: nVidia Strix GTX 960 (Brand new)

HD#1: Crucial 250GB SSD (Sata)

HD#2: Seagate 2TB (sata)

Blu-Ray: Generic HP drive. (sata)

Mem: 10gb (4+4+2), Kingston (Don't know the models, sorry)

ETC: 1 case fan, case has LED, memory-card reader, and extra USB/media connectors.

 

Once upon a time, I had a Corsair GS600w, and it died. I RMA replaced it, and couldn't wait for it in the mail and got a new PSU instead. The replacement Corsair GS600w was then used for another computer. An HP Pavilion gaming PC.

 

After installing it, the PSU would spin at 100% full speed as soon as the computer is turned on, then drop to 50-75% when idle. And constantly revs back up to 100%, when the computer is idling. Not doing anything.

 

Verified it is the PSU that's running this fast, NOT a buzzing or electronic noise, nothing electrical, and the operating temperature in the case us 40'C. It's currently 21'C in the room the tower is.

 

Is my replacement RMA GS600w in need of another RMA? Or is there a way I can throttle the PSU fan speed? It honestly sounds like a jet engine constantly spinning.