Reverse Gear Noise When Not Applying Throttle

[TheThing]

Apologies if this is the wrong place to post this. New to me car (2016 Mazda 6 Estate 2.0 Petrol) has just started making a sort of high pitched whirring sound when in reverse and letting off the clutch with no throttle. Anyone know the likely cause? It only started today. Thanks in advance.

[Baldchap]

Many cars have done that for decades. Don't worry.