Very faint whining noise

2009 Subaru Impreza. For a month or so I’ve been getting this very faint (but annoying) whining noise when on the throttle at approximately 40-60mph. It seems to only happen under load and it’s dependant on throttle position, not gear or RPM. I can’t recreate it in neutral or park because there’s no load.

Any thoughts?

FYI I’ve jacked up the high frequencies in this video and even then it’s still hard to hear. If you listen closely you’ll hear it.