That whirring noise is likely from the EVAP system of your Subaru. See below threads for more info about it. Having to replace your battery once per year is not normal though so there is definitely something else going on.

https://www.clubcrosstrek.com/threads/buzzing-sound-from-pas...

https://www.subaruxvforum.com/threads/whats-that-sound.14672...