I have that issue on my GL504GS.. same exact sound when the right side fan is on low speed. Sent in twice for RMA trying to get it fixed and they never replaced it.
I got the same problem and after searching a bit it seems the problem is internal misalignement caused by the force in the base of the laptop. It seems to happen more often when using a coolpad.
Anyway the only way to fix is to replace it. I´ve ordererd mines via Amazon in Europe and it is super easy to replace them. Just make sure you order the correct one.