4.8 is whining/whistling noise
 

Please take a look at this video: https://streamable.com/iaolr

The noise is starting after 17 seconds in this video when accelerating.

My car is a 2005 4.8is. I have already replaced the alternator which I though was causing the problem. However, now with a new alternator installed, the noise is still there.
I have also replaced the butterfly valve which did not change a thing.

Do you have any ideas?

I took another video today: https://streamable.com/cg3fb

Today I started the engine without the belt and the noise was gone, so it is coming from the alternator. However I have already renewed the alternator.

My 4.4 is having the same noise but relatively much diminished. I have to turn off audio, a/c and close all windows to hear it. Does yours make the noise when it's stationary or only when it's on the move?

From your investigations, the noise could also be coming from the pulleys or the belt, not necessarily the alternator itself. I would spray mist of water on the pulleys and belt to see if the noise goes away, in which case the pulleys/belt are the culprit and would need replacement.

Thanks for the reply!

Regarding when it's stationary, on idle you can hardly hear the noise, however when rpms increase the noise appears. In the first video, it is stationary for the first 15 seconds.

I have already renewed belt, pulleys and vibration absorber a few days before taking the first video. After that I have changed the alternator and took the second video after that.

Last week, I have used silicone spray on the belt however the noise did not change at all while doing that.

When starting the engine, the noise is not there for the first 2-3 seconds.

I think I may have fixed the whistling noise! I noticed that it emitted from the passenger side of the cabin and it was in sync with engine RPM. Also it was there when the vehicle is on the move or stationary. So I inspected and cleaned the pressure regulating valves and the cavity where it sits (# 13 in the diagram link below). The diaphragms were not torn but were dirty, especially the passenger side which had creamy stuff. Now no whistling noise!:thumbup:

RealOEM.com - Online BMW Parts Catalog

Take care not to break the tabs on the cover of the valves. I used a small flat-head screw driver to gently unlock the tabs I could reach and then gently pulled the cover off. When installing it back make sure you hear the tabs snapping in as you push the cover in place.

Good luck and hopefully this is also the root cause of your whistling noise.

The whistling noise came back with a vengeance, as if telling me :nanana:. But I think this time I've nailed it - the noise is clearly and most definitely coming from the speakers. I have a Dynavin N6 head unit. While the noise was there I rebooted the head unit, the sound disappeared. It came back right after the system was ready after reboot:whistle: I can make the noise stop by simply rebooting the head unit:D I still have the OEM head unit. Will install it back and see if that makes a difference.

Edit: Also noticing this happens only when Bluetooth is connected to my phone:dunno:

What you describe is ground loop whistle. Give it a GOOGLE you will see lots of suggestions on how to correct it. On my 4.8 it was the output transistors on the BM54 radio in the trunk.

Brilliant, thanks for the pointer Bruce! I think I know what caused it as I had recently worked on my audio system. I never used to have this noise.