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Thread: Hissing Noise

  1. #1
    Join Date
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    Hissing Noise

    So I need some help diagnosing a strange noise... Car is a 2002 325i w/ 205,000 miles, 100% stock. At first I thought it was just coming from the AC but the noise occurs even without the AC on. It is a hissing sound that is present at all times, except under acceleration. It almost seems to "cycle" at idle, if that makes sense. Weird thing is that when coasting, and very lightly tapping on the brake, the hissing lessens slightly with each tap, but doesn't go away completely if I am braking. The only time it goes away is under acceleration. I've got a couple videos for ya. The one from inside the car demonstrates the acceleration vs coasting.

    Inside car under acceleration, then coast: https://www.youtube.com/watch?v=-S_D7-Usndw

    Under hood @ idle with throttle input: https://www.youtube.com/watch?v=k4YM9YSiZD0
    CURRENT:
    18' F22 M240i | 6MT | MGM/CR

    02' E46 325i | 5MT | SBM/B (200k+)

    SOLD:
    10' E82 135i | 6MT | BSM/CR
    03' E46 330ci | 5MT | JB/B

  2. #2
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    2000 E46 328i
    Not really related to acceleration, but since it changes while braking; it might be a vacuum leak, possibly the brake booster.

    Does the brake pedal seem harder to press than normal?

    Edit: this seems to tell the same story: http://forum.e46fanatics.com/showthread.php?t=1018617 (faulty brake booster)
    Last edited by Raddee; 05-31-2018 at 08:44 AM. Reason: related page

  3. #3
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    Quote Originally Posted by Raddee View Post
    Not really related to acceleration, but since it changes while braking; it might be a vacuum leak, possibly the brake booster.

    Does the brake pedal seem harder to press than normal?

    Edit: this seems to tell the same story: http://forum.e46fanatics.com/showthread.php?t=1018617 (faulty brake booster)
    No change in performance/feel with acceleration or braking. It only slightly lessens the noise when tapping the brakes but then doesn't get any quieter from there. I did a couple quick stops from 50-20 and then 30ish to 5 or so, just for fun to see if that worked ha. Almost seemed to make it louder afterwards.

    I'll have to check out that thread. Thanks!
    CURRENT:
    18' F22 M240i | 6MT | MGM/CR

    02' E46 325i | 5MT | SBM/B (200k+)

    SOLD:
    10' E82 135i | 6MT | BSM/CR
    03' E46 330ci | 5MT | JB/B

  4. #4
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    Brake booster looks like a real pain in the *** to replace! Good luck..

  5. #5
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    I'm not sure if that's what it is. Reading through that thread it seemed like their noise got louder with the brake pressed. Mine is the opposite. But the noise seems to be coming from the same place. Also, I have no codes or changed pedal feel either... Hmm. I'll stay tuned to see if anyone else has any bright ideas lol. Luckily this isn't my car, and just a loaner until my M240i finishes production. But I'm trying to help the owner out with stuff while I have it. 16 year old kid just got it given to him by his dads work friend who was the original owner and put 200k+ on it. Pretty amazing. This is the only issue I've found in 2 months of driving it, other than one of the keys went bad.
    Last edited by kroptastic1881; 05-31-2018 at 09:19 AM.
    CURRENT:
    18' F22 M240i | 6MT | MGM/CR

    02' E46 325i | 5MT | SBM/B (200k+)

    SOLD:
    10' E82 135i | 6MT | BSM/CR
    03' E46 330ci | 5MT | JB/B

  6. #6
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    Disconnect the vacuum line from the booster and cap it off. If the noise is gone then it's the booster. Don't drive it like that though, you won't have power brakes.

    Sent from my SAMSUNG-SM-G870A using Tapatalk

  7. #7
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    Quote Originally Posted by randy72877 View Post
    Disconnect the vacuum line from the booster and cap it off. If the noise is gone then it's the booster. Don't drive it like that though, you won't have power brakes.

    Sent from my SAMSUNG-SM-G870A using Tapatalk
    Got a picture or diagram of what exactly I need to disconnect? Never done anything with the brake booster.
    CURRENT:
    18' F22 M240i | 6MT | MGM/CR

    02' E46 325i | 5MT | SBM/B (200k+)

    SOLD:
    10' E82 135i | 6MT | BSM/CR
    03' E46 330ci | 5MT | JB/B

  8. #8
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  9. #9
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    Quote Originally Posted by kroptastic1881 View Post
    Got a picture or diagram of what exactly I need to disconnect? Never done anything with the brake booster.
    There is a vacuum hose that goes into the booster near the master cylinder. It is a relatively large line (for a vacuum line) that goes to a check valve. The check valve is either straight or 90 degree and goes into a rubber grommet on the front of the booster (master cylinder side) and can be pulled out of the booster and capped/plugged. You can even have someone else put a finger over the end. There is also a suction jet pump in the line to the engine which can leak but it's under the hood. If you hear a vacuum leak INSIDE the car and the noise changes when applying the brake I would bet on a leaking booster. If the noise is gone with the vacuum disconnected it's confirmed. If it's still there try to locate by ear or do a smoke test. Do you have a CEL on and lean codes?


    Sent from my SAMSUNG-SM-G870A using Tapatalk

  10. #10
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    Pull out #3 (check valve) and cap/cover/block or use hose pinching tool on the rubber hose #2

    Sent from my SAMSUNG-SM-G870A using Tapatalk

  11. #11
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    Quote Originally Posted by randy72877 View Post
    There is a vacuum hose that goes into the booster near the master cylinder. It is a relatively large line (for a vacuum line) that goes to a check valve. The check valve is either straight or 90 degree and goes into a rubber grommet on the front of the booster (master cylinder side) and can be pulled out of the booster and capped/plugged. You can even have someone else put a finger over the end. There is also a suction jet pump in the line to the engine which can leak but it's under the hood. If you hear a vacuum leak INSIDE the car and the noise changes when applying the brake I would bet on a leaking booster. If the noise is gone with the vacuum disconnected it's confirmed. If it's still there try to locate by ear or do a smoke test. Do you have a CEL on and lean codes?


    Sent from my SAMSUNG-SM-G870A using Tapatalk
    Quote Originally Posted by randy72877 View Post
    Pull out #3 (check valve) and cap/cover/block or use hose pinching tool on the rubber hose #2

    Sent from my SAMSUNG-SM-G870A using Tapatalk
    Awesome info. Much appreciated!

    After leaving work last night, I did some more "testing" and the hissing completely goes away while stopped with the brake pedal firmly pressed. I think you guys nailed it on the brake booster. I'll do some troubleshooting that randy laid out and keep this thread updated for any future searches. Thanks!

    Oh by the way, I do NOT have any codes being thrown. The hissing just started probably 2 weeks ago or so. I had just assumed it was the AC since its been on practically full blast since then.
    Last edited by kroptastic1881; 06-01-2018 at 08:44 AM.
    CURRENT:
    18' F22 M240i | 6MT | MGM/CR

    02' E46 325i | 5MT | SBM/B (200k+)

    SOLD:
    10' E82 135i | 6MT | BSM/CR
    03' E46 330ci | 5MT | JB/B

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