Hi there, ever since I did a supercharger service as well as replaced my water pump on my R53 a couple of months ago it has had this whirring noise. The noise was most audible at idle and it sort of sounded like a PlayStation (loud fan going off). Recently I have just done a 17% pulley upgrade and has resulted in the noise being the same at idle but much more present during acceleration. The noise now rises with engine revs and is much louder than before. It really sounds like a rising whir sound or rising fan sound but it’s definitely not the supercharger whine haha. I suspect it’s either one of the following two things: perhaps a vacuum leak somewhere in the supercharger system or more likely the water pump which was perhaps defected from new and causing it to be a bit loud (since the pulley drives the pump it would make sense that the noise got louder after doing the reduced pulley). Let me know what you think, cheers.

 

This old thread was just revived Mysterious whine (not the typical supercharger whine) after engine maintenance , which might be of interest.
Although the original poster left us hanging without a resolution to their problem - I might suspect, perhaps, possibly the alternator, AC clutch or from my own personal experience - a failing idler pulley.
While my idler pulley began to fail; it sounded like a high pitched "tiny supercharger" whine. The high pitch sound raised and lowered when accelerating and a little quieter with deceleration until/before the idler pulley finally detonated.
You might want to start by removing the serpentine belt and hand spin the idler pulley. Inspect/turn the other pulleys as well. You may also want to start/run the engine for a minute or so while the belt is off to hear if the whine is no longer heard. That may help in deducing the potential failing point/pulley.

 

Yeah that might be a good shout except when I did this supercharger service as well as replacing the water pump I also replaced the serpentine belt tensioner and the idler pulley which is unusual for it to already be potentially failing. I may have been given a bad idler pulley out of the box so I will check it in the coming days to see if that's the problem. I seriously doubt the noise is AC or alternator related as the issue only began since doing that big job. I will work on the car tomorrow and reply back with what the problem was or may be for any other people who are interested, cheers.