PSU whining in sli

hi there, i recently added another gtx 560ti to my system and when the graphic cards are at full load in games like planetside 2 the psu makes a whining noise. it isnt too loud and it only happens when running graphic intensive things.
it is definitely the cpu as i tested each of the gpu's individually and the problem only occurs when running both of them.
750w should be enough to power this system. anyway please help, should i rma my psu? or will the next one do the same thing? is the whining safe? will it eventually die?
the psu in question is the Antec True Power New Continous 750W Modular Power Supply (http://www.scan.co.uk/products/750w-antec-true-power-ne...)

here are the rest of my system specs

asrock z77 extreme 4
3570k
asus 560ti, gigabyte 560ti
8gb ram
3hdds
1 optical drive
1 internal sd reader
4 fans
this is all thats plugged into my system