We did the exact thing not too long ago, but we couldn't do it using blue/green. We were able to minimize downtime quite a bit, but it was on the order of minutes, not seconds. I wrote a little bit about the process here. I spent a lot of time prepping and running the migration, so happy to share any details if it's helpful.
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://docs.fileformat.com/gis/fit/