High-pitch noise on 135i [solved - exhaust valve vacuum hose]

[Poptarts]

Hi guys,

Some time ago I experienced a very annoying noise that would start as I put load on the engine (idle = silent, appears as you start driving).

It was kind of hard to localize, seemed to come from the dash. I managed to narrow down the frequency of it using a sine tone generator on my phone, it was around 9kHz.

It was pretty directional so if you moved your head a bit it would be more or less noticeable. But overall very noticeable (even my wife was bugged and she usually doesn't pay attention to noises).

On cold starts it would make the noise for about a minute at idle then disappear.

Given all these clues I highly suspected the controller of an electric motor or pump (they use switching at high frequency) even though it didn't make sense why it would suddenly change its operating frequency after years working normally.

Well, I finally found the issue!

One morning I let the engine idle and had to grab something in the trunk. So I went out, opened the trunk and noticed the noise was there too, although it sounded a bit more like white noise.
Then I walked around he car, near the ground, and noticed the sound was very loud by the rear left wheel...

Guess what?! When I had the stock muffler I had disconnected the exhaust valve and pinched the vacuum hose. Then I installed a Berk Street and for some reason I removed the zip tie (maybe I was thinking about pulling the hose into the trunk then forgot about it...

In the end it was whatever mechanism that is used to create the vacuum (my guess is a venturi that starts somewhere behind the dash) that was making all this noise. When I pinched the hose it went away immediately.

I didn't notice any bad side effect when this happened (no code, everything behaved normally).

Bottom line: I feel pretty stupid, super happy it was that simple, and if this can help someone then it is posted now!