This sounds distinctly like alternator whine. Perhaps a audio signal wire is too close to a power cable. Or this may be caused by a ground loop. Improper install of aftermarket stereo/speakers/amplifier?
Disconnect the speaker harness from the radio. Start the car and verify the whine is gone. Check that no speakers are shorted to the chassis, if so, there is a ground loop.
Then try unplugging individual inputs and accessories.
And lastly the radio antenna.
I shall try this, I have the same problem…
2006 Legacy 2.5i