Seasonic x 660w has a low coil whine

Just finished building my computer with a new Seasonic X660w PSU. But when I put my ear very close to the power supply, I can hear a very low and constant buzz. My computer was shut off but I left the power on for my PSU. And I'm sure the buzz did come from the PSU not from my motherboard or graphics card.

Read some other forums and people say Seasonic or some other PSUs will always have this low buzz depends on how close you put your ears there. So I'm wondering are there any PSUs are absolutely quiet. The noise doesn't bother me because I'm sure I'm not able to hear a thing if I put the case underneath my table unless I use it as my pillow every night. But will it become a problem in the future? Or should I just get a RMA? If you guys have a similar problem, I'd like to hear your opinions.

Thanks!