I have a 2005 Cooper S with 66k miles. Recently I have discovered that it has been making an alarming ringing/rattling noise from the engine bay. It only comes on above roughly 2500 rpm when you are accelerating or under load. I can only hear it when the windows are down.

Here is the quick and dirty of what I have done to isolate where the noise is coming from.
-Removed the serpentine belt from the motor and ran without accessories (could not get the car to make the noise, which leads me to believe the rattle is somewhere in the accessory drive.)
-Rotated supercharger, ac compressor, and alternator by hand while belt was off. All felt smooth, but there was slight rotational play in the supercharger.
-Replaced the crank pulley (have heard this can cause noise, mine was fine, but I replaced it just in case.)
-Replaced belt tensioner, idler pulley, and belt
-Replaced intercooler boots (in case it was a boost leak, they were cracked.)

At this point I have narrowed down the noise to either the supercharger and waterpump, the alternator, or the ac compressor. I am leaning toward it probably being the supercharger waterpump gear, but I was curious to know if anyone else had had this problem with their car.

 

The water pump usually makes a knocking noise.

Does it sounds like this?

https://www.youtube.com/watch?v=e8WV...ature=youtu.be

 

Harmonic balancer? I had to replace mine. Similar symptom.

 

In response to this older post -
My 2wo previous experiences with this/a similar noise resulted in the idler pulley detonating/failing. The noise almost sounded like a higher pitch supercharger whine or high pitch ringing under load.