Sound coming from my external iPhone battery?

This may seem like an awkward question, but since when did batteries make a whirring sound? It's not like they have movable parts inside like a hard drive. I'll post the link to the exact external battery later.

IPhones don't utilize HDD. Their method of data storage and extraction is flash storage, not unless you have the iPod classic. I hear they have 2.5 HDD's inside of them. Same ones used in laptops that still use the HDD's and not SSD's.