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Originally Posted by Lotus99
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Originally Posted by ///Mposter
Sound like the boost solenoid. It would make sense that it happens when you press the accelerator because it takes input to the solenoid and allows vacuum past it. When they go bad it vibrates. In my case mine would vibrate at idle loudly like a buzzing sound . I wonder if I still have it on video. They replaced it under warranty but I had to show a video of working normal and one where it was buzzing.
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Hi, Who are you addressing? If it's my noise, I wouldn't think so, because the noise is not there during idle, or at all RPM's even, only within a certain range of RPM and when the engine is cold. After 5 minutes, the sound completely goes away, and if it was a boost solenoid, why would it go away when the engine is warmed up?
This is my issue:
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Originally Posted by Lotus99
Unfortunately the sound is back in my case. Here's what it's like. Happens on cold starts only, and only for the first five minutes of driving, then it goes away. 
It is most noticeable when going up a hill when there is a load on the engine, and generally between 3000 and 4000 RPM.
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Hi , I'm addressing you. I have had different sounding noise on my 335i than the X3 . Both were boost solenoid a. On my 335i it sounded like it was coming from the radio or behind the dash and was intermittent. With the X3 it was at idle and would go away as the pedal is pressed. I couldn't say that there is a specific reason why yours is different but maybe if you could have someone try to rev it while you have the 5 minute warm up you state it happens;maybe you will hear something.
I was also surprised at how much it was moving/buzzing once I did locate it on the X3.
Also electrical items tend to work different in warmer temps than colder. Coils are a great example. Possible misfire at warm up/ start up but fine after a drive cycle.
Well whatever the case is, I hope you are able to find the noise. Maybe a go pro under the hood would help.
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