Whats making noise

[Andrew_263]

Hey so I built a new pc.
Nzxt s340
Gigabyte z170
Evga gtx 1080 oc
Corsair h115i
16gb 3200mhz ram
I5 7600k
Evga 750w modular psu
And 2 WD 1tb hdd

Now I only just noticed this revving noise on and off in intervals of a few minutes. and only just now when I started transfering data between external hdd and usb. I don't know if it happens during gaming... and my cpu temps never exceed 65c° even after hours of high graphics high intensity gaming.
Now any idea the cause? Reasoning behind? Etc
I'm pritty sure transfering data is not that hard on a pc.
Thanks in advance

 

[helpstar]

probably the HDD which wakes up/ goes to sleep?