New build , weird buzzing sound.

So i finally set up my new build and everything runs extra smooth apart from one minor

yet very annoying thing. I hear a low but high pitched sound from inside the case and as

far as i can tell it's fan-related. I tried stopping the case (nzxt 410 phantom) and gpu fans

while working but that didn't stop the noise so it is probably the cpu heatsink. It is a new

i5-3570k fan that i just installed. I already tried re-assembling everything yet the sound is

still there , also ran bios and lowered fan speed a little bit and that did indeed lower the noise

but it is still noticeable. Do you guys think i should replace it with an aftermarket one, or is

there an easier solution that i didn't think of. Right now i'd rather get a 40 euro heatsink like

hyper evo 212 than sending back the whole cpu just for a fan replacement since it is the only

pc i have in my place. Thank you in advance.

Edit : motherboard is an Asrock z77 extreme6 / ram : 8gb corsair vengeance / gpu : 7850 OC sapphire 2gb

PSU : corsair tx650-m