Radio to alternator, I read you loud and clear!

[BennyM]

So I've owned this 525i for 3 years now and it wasn't until today that I figured out where an annoying high-pitched whining noise was coming from. It sounded like a whistle from a vauccum leak and changed pitch with engine speed. But over the years, I've replaced everything that could leak. Some days it was louder, some it was quieter. Finally, I realized that the noise goes away if I turn off the radio!

Now, I had this same Nakamichi deck wired the same way in my 92 e34 with no weird noises, so I'm 80% sure it's not a wiring issue. I also read in the archives that there's something attached to the alternator called a "radio noise suppressor." Does that sound like a likely culprit? I know the alternator makes some strange scraping sounds from time to time and could use an overhaul, so maybe that little suppressor thing has already died.
Thoughts?

[Tiger]

99% of the time of alternator whining noise is poor ground contact... at the head unit and the amplifier. By using the factory ground wire, it should be perfect... no noise. If you ground the wire to the chassis direct... be absolutely sure you bare the metal... paint is resistance and causes alternator whine noise.

[whiskychaser]

+1
This guide is pretty good if you are still struggling:
Noise Suppression Guide

[Ross]

"Finally, I realized that the noise goes away if I turn off the radio!"
Sorry, Benny but I'm still laughing at this one.
Funny too because my wife uses the opposite method regarding car noises and the radio.

[BennyM]

"Finally, I realized that the noise goes away if I turn off the radio!"
Sorry, Benny but I'm still laughing at this one.
Funny too because my wife uses the opposite method regarding car noises and the radio.

Haha! I can see how that would sound pretty obvious taken out of context.

[BennyM]

So, it's either a weak ground or a bad noise filter. I'll try to check the wiring this weekend, but it's wired (grounded) through an aftermarket to factory adapter, which seems pretty sturdy and straightforward. So maybe I should add an additional ground?

[whiskychaser]

A bit late in the day but the suppressor you mentioned is probably part no 12311356564. It looks like the alternator design changed 9/91 and it was no longer required.