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Thread: Low-RPM whistle noise only sometimes

  1. #1

    Low-RPM whistle noise only sometimes

    As the title suggests, at low rpms, there's a slight whistling noise coming from the front. Weirdly enough, when I got it to the shop today, they couldn't replicate the sound. Research is hinting it being a lower intake boot cracked, but I do not have any lights coming up either.

    Does anyone have any experience with this kind of issue? Anything would be greatly appreciated.
    1991 E30 Alpinweiß 325iX [SOLD]
    2005 E46 Sparkling Graphite Metallic 330Ci ZHP 6spd

  2. #2
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    Check the 3 vacuum caps on the back of the intake manifold. There are 2 small ones and 1 larger. Both are made out of rubber

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    His - 2004 Imola Red / Alcantara 330i | 6MT | ZHP |

    Her's - 2005 Black Sapphire Metallic / Black Leather 330i | 6MT | ZHP |

  3. #3
    Thanks for that, didn't know that was even a thing. Do you mind giving me the rundown of how I can find this? All my research is kinda explaining to me in a third language. Would it also be a decent idea to remove the air box anyways to check for cracks in the intake boot?
    1991 E30 Alpinweiß 325iX [SOLD]
    2005 E46 Sparkling Graphite Metallic 330Ci ZHP 6spd

  4. #4
    Join Date
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    Part number 11611437560 - Quantity 2 needed
    Part number 11611727176 - Quantity 1 needed

    These caps are used to cap the extra vacuum ports that are not used on our cars. To access them you need to remove the cabin air filter/assembly.

    In regards to the intake boots, you can feel how hard they are. Newer boots are pliable while older ones close to the end of their lifecycle are hard.

    His - 2004 Imola Red / Alcantara 330i | 6MT | ZHP |

    Her's - 2005 Black Sapphire Metallic / Black Leather 330i | 6MT | ZHP |

  5. #5
    Join Date
    Jan 2013
    Location
    Central California
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    Here's a pic of the three vacuum caps on the back of the intake manifold.

    Name:  Vacuum Cap Intake Manifold D7.0 1727176 3 WM.jpg
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Size:  63.9 KB

    If it's not that then try your best to localize the noise. Maybe it's a fan belt pulley? I think the SAP can also make a high pitched noise.

    It always a good idea to use a scanner to check for codes. If the noise is caused by a vacuum leak you would likely have P0171 and P0174 codes.

  6. #6
    Can't tell you definitively, but I've had that whistling on two separate occasions in my car, and in both cases it was indeed a cracked lower intake boot. For me it was a very high pitched whistle/whine, barely audible but still super annoying. It was only audible at low RPM **and** barely-any-throttle. If you floored it (either at 1k or at 5k) the whistle wasn't there. Not sure why at 1k. At 5k you certainly wouldn't be able to hear the whistle over the rest of the car noise (at least the way my whistle sounded). Not sure it that might help you or the shop replicate the symptom on demand.

    2004 E46 330ci ZHP Coupe | Mystic Blue/Black Leather

  7. #7
    Join Date
    Dec 2010
    Location
    Fairfax Station, VA
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    2,806
    Easy test for any vacuum leak, any car, get a can of spray carburetor cleaner. With the engine running, spray the carburetor cleaner around suspected vacuum leak areas ((e.g., the 3 caps, lower intake boot, etc.). If the idle suddenly rises, you’ve found the area of your leak. The suction of the intake system will pull in that carb cleaner, its very rich and will raise your idle for a second or two. Then get more precise with your carb cleaner sprays and pinpoint your vacuum leak. Hope that helps.

    Bruce
    21 GMC Sierra - mine
    16 X5 M-Sport - wife
    11 M3 - 1st son
    04 Tundra - 2nd son
    17 X1 M-Sport - daughter

    Former (mo/yr sold):
    15 335xi M-Sport (8/18)
    11 335is (8/17)
    11 X5 AW/blk (11/15)
    04 330xi blk/gray (7/15)
    06 ZHP SGM/blk (6/15)
    08 335xi blue/blk (4/15)
    04 ZHPci MB/blk (2/14)
    04 ZHPcic IR/Alc (10/13)
    01 540iT anthra/blk (7/13)
    04 ZHPci TIAG/blk (9/12)
    03 540i M-Sport (9/12)
    01 325ci (3/12)
    02 325iT 5MT (10/11)
    05 ZHPci (7/11)
    03 330cic (2/11)
    00 323ci (10/10)

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