Grating noise when power steering is not engaged

Car emits a low grating humming noise coming from the driver's side engine area roughly behind the air box. Noise goes away completely if the steering wheel is engaged (turned). Noise happens while parked or driving and stops as soon as you turn the wheel in either direction.

No noise where the power steering reservoir or pump are (passenger side). Power steering working fine with no noise while turning.

Please help. Thank you.