A quick video Comparing idler (deflection) pulley noise BEFORE and then AFTER a new one has been installed on a BMW 330i (E46)
Here’s a DIY on how to replace your bad idler pulley:
You should replace your main Serpentine Belt at around 50-60K, so while the belt is removed… check your pulleys (Both the Idler AND Tensioner). Give them a quick spin and listen to how they sound.
If they sound like anything like this video, definitely have them replaced.
You should replace both pulleys at the same time, if possilbe.
This DIY was performed on a 02 BMW E46 330i automatic, alpine white paint, with Sport Suspension Package, you know the drill. So if that is not your vehicle, there might be differences.