Could be
the in-tank fuel pump.
Some people have had it replaced, some have only had a software update. Neither of these options seems to work well. I had the sound also and after a software update it got a lot quieter, but I feel it's been getting worse again lately.
After the software update I noticed that when I unlock the car or open the trunk there's a 3-second humming sound coming from the back. This happens only in the mornings or when I haven't driven the car for hours.