mudspike

E60 N52 engine…
Got O2 sensor codes… 2C31, 2C6B
No warning light popped up, no power loss, no shaking so far but I found this codes during regular check-up which I do every other month.
Looking at the error history, error seemed to pop up several times over the past couple month..
I do not believe that two sensors went bad simultaneously.
So I suspect that the other part which can affect air/fuel ratio or exhaust gas quality is failing…

I am not sure if it is correlated but recently I am hearing a whirring noise from engine bay.
It’s heard all the time.. when the car is in motion….cruising, accelerating.. low/high speed.
The noise sounds very similar to that car’s making when the engine is under down-hill engine braking… low pitch whistle

NO noise is heard when the car is stationary and the gas pedal is pressed or idling…

Any suggestion before I change expensive (2) O2 sensors?