Electric motor sound under trunk area after engine shut-off?

[youngsj]

It's pretty pronounced and kinda sounds like a medium speed electric pump. It doesn't happen all the time (~4-5 times over the last 1,000 miles), but it's usually after I've pulled into the garage after coming home from a spirited 45 minute commute home on back roads (under 5k rpm and no WOT).

I'll pull in the garage, shut her off, get out and hear the noise coming from somewhere in the way back of the underside (sounds like it's coming directly dead center under the trunk), but I haven't been able to find the exact source yet.

Maybe it's the fuel pump (I admitedly don't know wher the fuel pump is located in this car)...but it is pretty loud and I'm not sure why this would stay running for awhile after shut off? I also haven't stuck around more than 5 minutes (I usually walk out of the garage with the noise still going with a puzzled look on my face), so I don't know how long it keeps going after 5 minutes.

I've searched alot and read a few posts about partially separated fuel tanks needing fuel balancing across the sides...and having a pump to manage this process - but yet I didn't read anyone describing this noise. I also have a bit of rough engine until fully warmed up (minor jerky acceleration)...and I've read posts on faulty (or failing) fuel pumps running dry that maybe cause this mildly rough ride during the first 5 min of engine warm-up, but I'm not too worried as it will get worse and simply need a quick dealer replace if this is the cause.

Anyway, I was just wondering if others hear this electric motor noise after engine shut off and can tell me what this sound is?

[mhutchins14]

I was just about to ask the same question. I have 1100 miles on my car and I heard this same sound for the first time the other day. Like you, I examined the underside of the car and it seemed to be coming form underneath near the differential. I cannot remember if the car has some sort of cooling device for the rear diff.

[ybbiz34]

Take a deep breath, relax. It is probably no big deal. Although, I can't be too critical, I tend to get a little paranoid myself sometimes.

[youngsj]

Yeah, I'm not too worried about it.

I actually first thought it was simply the timed auto-venting that you can set in the iDrive settings to vent the car at certain times of the day/night...but it's not that, cuz I checked to be sure all that was off.

It'd just be cool to know what it was...

[oneginee]

http://www.m3post.com/forums/showthr...33#post2532233

the fuel pumps are located in the rear of the car by the tank.
If one pump is running dry it will tend to overheat and wear out quickly. Plus you don't want your engine running uneven cycles because of a fuel pump selector valve problem or fuel regulator leakage.

go have a mechanic check it out. It is easy to diagnose without doing any work on it.

[MysticBlue]

My M coupe does that and my previous 330i did it. It's the fuel pump. Nothing to worry about. Although I have never understood why sometimes it does it and sometimes it doesn't.

[skierman64]

I've heard the same noise, but it doesn't sound muffled as if it was coming from inside the fuel tank. It sounds simliar to the motor on a automatic retracting radio antenna cars used to have. Or like a fan of some sort.

I'm not concerned, but curious.

[youngsj]

Skierman64 - Your description is exactly what I hear too.

MysticBlue - Glad to hear you've heard this on your other 3 models as well.

Oneginee - Thanks for sending the link...I had actually come across this thread in my previous forum searching (described in my original post).

I'll get this checked this week while at the dealer for my 1st oil change - I scheduled an appt and went in last week and they kept me waiting for 3+ hrs and then told me they could only change the diff fluid and air filters cuz they didn't have oil filters in stock (same experience that I've read some others are having on 1st oil change).

As an interesting side note - My first appointment was triggered by a call to my house from my dealer saying my car had phoned in to request service (my son took the call while I was out running errands)...but I only had ~600 miles. I've read that a few others have had this call from the dealer, but not the norm. I know I'm nit picking...just seems a little strage.

Anyway - Thanks for the responses guys

[visor]

Yeah, it's the fuel pump doing its priming thing.

Does it also happen right after you unlock the car with your remote keyfob?

[kbk_75]

Strange...can't say I've ever noticed this sound either at start-up or shut-down on my car!

[pilot_bro2003]

My previous BMW 2006 530 did the exact same thing as well! I never worried about it and just figured it was the car doing its own thing. It seems like I remember it would make that noise too when after sitting over night and I would unlock the car...

[ersin]

My E46 M3 SMG did this when I unlocked the doors too. But that was the SMG pressuring up.

[DJ9]

Ironic enough, I noticed this for the first time last night. I dont think it's anything to worry about.

Funny, i have 700 miles right now, car reads 400 miles to go unti oil service, and I guess there is an Auto Request feature in iDrive that has already scheduled my service appointment for me!

[badfish]

I took the trunk apart yesterday and there's this large silver box that is near the seats. I didn't investigate it at all but I think that it's probably the source of the noise. See picture below. It's the silver box just to the right of the center of the picture.

[pilot_bro2003]

I wonder what that is!?!?!?!?!

[AMPowerJ]

I just heard this tonight for the first time. Did anyone ever figure out exactly what it was and why it sometimes happens? I think it is the fuel pump as well but it would be cool to understand what it is doing.

[M3MIKE]

The sound is the fuel pump. My M6 does this when I unlock the doors and sometimes randomly when I'm washing the car. If I'm washing and doors are unlocked and I spray door handles with water you hear the fuel pump kick in and door lock. Comfort access is very annoying!

[monk]

Originally Posted by badfish

I took the trunk apart yesterday and there's this large silver box that is near the seats. I didn't investigate it at all but I think that it's probably the source of the noise. See picture below. It's the silver box just to the right of the center of the picture.

Could be a rat under that silver box.

[JCtx]

My guess is that silver box is a stereo amplifier.

And I also hear that noise when I open a door, even without the CA remote in my pocket. Weird.

[Race_Doc11]

+1

[Technic]

Originally Posted by badfish

I took the trunk apart yesterday and there's this large silver box that is near the seats. I didn't investigate it at all but I think that it's probably the source of the noise. See picture below. It's the silver box just to the right of the center of the picture.

That's the Assist TCU... there is no moving parts inside.

Now, there is a fan to cool the old Sirius tuner (another silver box to the left of the TCU-up to MY2008, not shown in this photo) that sometimes stays on after the car is shut off.

[J08M3]

I don't have sirius and my car makes this noise too. it does sound like a fan, not a fuel pump priming the car as the sound you get before you start it.