I just build a new system and used nEVGA NEX 750G PSU.  When the system powers down the PSU makes a horrible noise.  It's hard to describe, but it almost sounds like something is hitting something else.  It's a very audible clunk.  It last's about 2-3 seconds.  What's strange is that when the system is powered and running it doesn't make the noise; however, I also haven't had the system running for more than 5 minutes yet.  I didn't see the point in moving forward with such a horrible sound coming from the PSU.  I also notice a slight hum from the PSU while the system is running though, but it's not nearly as audible as the power down clunking sound.  Has anyone ever encountered this issue with this PSU before?  Any suggestions?  I am considering returning it at the local store I purchased it from.  It makes me sad though, this is my first issue with an EVGA product...knock on wood!
 
Also, when the system is at low speeds, such as in the bios, the PSU also makes the same loud noise!
 
Full System Specs:
Corsair Obsidian 750D Case
Corsair H100i CPU Cooler
Gigabyte Z87-UD4H Motherboard
Intel i7 4770K CPU (stock speed)
8GB (2 x 4GB) G. Skill Trident Series 2400MHz CL10 Memory
ASUS Direct CUII GTX680 2GB Video Card
240GB Intel 530 Series SSD
1TB WD Black Edition HDD
LG 12x Blue-ray Burner
 
Thanks!