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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Buzzing Sound Form Rear of Car



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      03-26-2007, 01:15 PM   #1
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Buzzing Sound Form Rear of Car

Has anyone heard a buzzing/humming sound coming from the rear of the 355i after the car has been turned off. I've heard the sound a few times it does not show up all the time. you can hear it if you bend over near the rear wheels. It sound like a small motor is running. I was wondering if it was the gas pump, which I am assuming it is in the gas tank.

It is not very loud, and I hear most often in my gargage with minimal background noise. I know it is not the turbo spinning down, I know what that sounds like, this sounds more electric. After a few minutes it stops.

Last edited by Maestro; 03-28-2007 at 08:30 AM.
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      03-26-2007, 02:17 PM   #2
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Quote:
Originally Posted by Maestro View Post
Has anyone heard a buzzing/humming sound coming from the rear of the 355i after the car has been turned off. I've heard the sound a few times it does not show up all the time. you can hear it if you bend over near the rear wheels. It sound like a small motor is running. I was wondering if it was the gas pump, which I am assuming it is in the gas tank.

It is not very load, and I hear most often in my gargage with minimal background noise. I know it is not the turbo spinning down, I know what that sounds like, this sounds more electric. After a few minutes it stops.
Yup, I had the EXACT same noise the other day (was the first time I heard it after 4 months of ownership). It was coming from the right rear underbody. I had run it somewhat hard for about 10 minutes. It sounded like a small motor just like you say. It's not loud but definetly audible if everything else is quiet. It lasted a while 2 or 3 minutes and I just walked in my house as this car is ALWAYS making weird noises after shutting it off. All of the noises seem normal, though. I just thought this sound was another of its noises.

This noise sounded different, though, kind of like it was a bit out of line. It might have something to do with the fuel pump although it could be anything.
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      03-26-2007, 02:42 PM   #3
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you should really let her out of your trunk man
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      03-26-2007, 03:22 PM   #4
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you probably hear the fuel pump
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      03-26-2007, 04:17 PM   #5
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I belive its the fuel pump since one of them is located under the passenger side rear seats put your ears up to the rear seats butt cushion and give it a listen
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      03-26-2007, 05:18 PM   #6
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Same thing here about 3 weeks ago - low tone buzz for about 20~25 seconds from right near the filler access door.

I guessed it was the fuel pump or fuel return running, but thought it odd that I had never heard it before...
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      03-26-2007, 08:38 PM   #7
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Its the DMTL pump running a tank leakage test... its normal...
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      03-26-2007, 11:19 PM   #8
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Quote:
Originally Posted by 1clean335i View Post
Its the DMTL pump running a tank leakage test... its normal...
Gee, how the hell are we all suppose to know this stuff... If it wasn't for this forum this kind of thing would drive me nuts.

But why would this do this test after you shut the car off.

I think I have a problem that things are still on and running after the car has been turned off... call me old fashion this does not see right.

Ok, maybe the reason I do not hear it all the time is that it seems more noticeable on a low tank, more space to fill.

Last edited by Maestro; 03-27-2007 at 12:22 PM.
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      03-27-2007, 11:40 AM   #9
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Quote:
Originally Posted by Maestro View Post
Has anyone heard a buzzing/humming sound coming from the rear of the 355i after the car has been turned off. I've heard the sound a few times it does not show up all the time. you can hear it if you bend over near the rear wheels. It sound like a small motor is running. I was wondering if it was the gas pump, which I am assuming it is in the gas tank.

It is not very load, and I hear most often in my gargage with minimal background noise. I know it is not the turbo spinning down, I know what that sounds like, this sounds more electric. After a few minutes it stops.
I hear a sound very similar to what you're talking about in my garage when ever I open the drivers door. Again after you shut the door. Then if I stand in the garage long enough like 4 minutes, I hear the sound again for about 30sec. Hmmm.... It does this everytime. Again you have to be in a quiet garage to hear it.
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      03-27-2007, 06:06 PM   #10
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There's also a fan that cools the amp for the stereo, that's on the left side of the trunk. That too will run for a bit after the car is shut off.
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      03-27-2007, 06:45 PM   #11
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Quote:
Originally Posted by Maestro View Post
But why would this do this test after you shut the car off.
I would think it needs to pressure test the tank with the fuel pump off so it isn't losing any pressure through fuel being pumped out.

Quote:
I think I have a problem that things are still on and running after the car has been turned off... call me old fashion this does not see right.
Modern cars have so many emission control/convenience features that require time after shutoff to sort through everything. Your car is probably still doing stuff 5 minutes after you exit the vehicle. It's the way things are headed these days.

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Ok, maybe the reason I do not hear it all the time is that it seems more noticeable on a low tank, more space to fill.
That seems like a reasonable guess.
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      03-27-2007, 07:36 PM   #12
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i think u guys are talking about the fan on the stock amp. it stays on for a while till the radio goes to sleep. next time it happens, open the trunk and remove the plastic cover on the amp.
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      03-28-2007, 09:03 PM   #13
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Quote:
Originally Posted by Maestro View Post
Gee, how the hell are we all suppose to know this stuff... If it wasn't for this forum this kind of thing would drive me nuts.

But why would this do this test after you shut the car off.

I think I have a problem that things are still on and running after the car has been turned off... call me old fashion this does not see right.

Ok, maybe the reason I do not hear it all the time is that it seems more noticeable on a low tank, more space to fill.

lol its all good man... i just happen to be a tech so its no big deal... imo, its better to inform u guys about things like this rather than having customer bring it into the shop and waste their/my time......

the car cant run the tank leakage test with the car running so it does it everynow and then when you shut the car off... weird but true.. just like sometimes the electric fan will run when the car is shut off... if you have a 335i you might even hear the water pump running to circulate the coolant through the turbo's so they dont overheat themselves
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      03-29-2007, 12:39 AM   #14
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Quote:
Originally Posted by 1clean335i View Post
Its the DMTL pump running a tank leakage test... its normal...
hehe I was about to say the same
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      03-29-2007, 08:58 AM   #15
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Quote:
Originally Posted by 1clean335i View Post
lol its all good man... i just happen to be a tech so its no big deal... imo, its better to inform u guys about things like this rather than having customer bring it into the shop and waste their/my time......

the car cant run the tank leakage test with the car running so it does it everynow and then when you shut the car off... weird but true.. just like sometimes the electric fan will run when the car is shut off... if you have a 335i you might even hear the water pump running to circulate the coolant through the turbo's so they dont overheat themselves
Thanks for that information, I appreciate it. It is great to have this kind of knowledge.

Yeah, I had other cars that the fan ran for a few minutes after the car shut off and that too bothered me. It tells me that the car was poorly design, it was like it was barely keeping itself cool. Those were not performance cars and I can see a performance car needing continue cooling after shut down.

I did not think it was the fan on the power amp since it makes no sense because as soon as you turn off the power the amp would begin to cool down. Unlike an engine which the cylinder are still much hotter than the rest of the engine and might still need water circulating to bring their temp down.
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      06-22-2007, 09:17 PM   #16
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I actually had my DMTL pump fail the other day. Got the "Service Engine Soon" warning lamp, brought it in and they said it was the DMTL pump. I had no idea the car had a pump just for diagnostics, crazy stuff.
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