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Old 04-01-2014, 04:41 PM   #1
holycrapitsjer
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Join Date: Apr 2014
Location: Southern California
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My Ride: 2003 325Ci
Arrow Rear-end *THUNK* while braking...

Hey all! Long time lurker, first time poster here! I've got an early-2003 325Ci with 130,000 miles thats recently developed a thunk-like sound as well as a strange power transfer-like feeling within the last few seconds of braking. I've searched these forums as well as other sources online for an answer with no luck so I thought I'd try making my own post and seeing if anyone could help. I'm aware of the dreaded subframe tearing issue but feel that the symptoms I'm experiencing don't sound exactly like they had so I'm hoping for the best!

Here's a quick rundown of what happens:
When braking semi-hard from anything 30mph or faster, between 5 and 0 mph, there's a "metal-on-metal" clunk/thunk sound that seems like it's coming from the rear of the car. When this happens, the RPM gauge as well as the MPG gauge "flutter" (for lack of a better word) and seem to have a direct correlation to the strange "power transfer" feeling. It's almost as if something along the drivetrain wants to keep spinning at the speed it was (or a little faster than it should be given the car's coming to a stop) but is stopped abruptly by the movement of a possible loose part (the thunk).

A few things to note:
- This ONLY happens when braking from 30mph+ and not when changing gears from park/reverse/drive/sport/manual mode.
- This happens EVERY TIME, in what seems like the same spot timestamp-wise (if that makes any sense), without fail.
- I'm able to somewhat "fight" the thunk from happening by braking softly right up to the point where I can feel it about to happen (judging by the "power transfer" feeling), easing up off the brake pedal for half of a second (when the thunk would normally happen), then pressing it down hard again and stopping the car.
- This DOES NOT happen if I drop into neutral while braking and doesn't make any sort of sound when changing it back to drive and accelerating afterwards, leading me to believe the problem is at least partially-mechanical and lies transmission -> back.
- There's no check engine light or codes to be pulled.
- Besides this, the car seems to be running strong and I'm happy with it otherwise (except for just about every miscellaneous ABS plastic part breaking right around the 10 year mark... damn manufacturer obsolescence!)

Has anyone else experienced anything similar? Could this be a symptom of everyone's favorite torque converter issue? Rear differential bushings? I've got tools and garage space and could do any investigating necessary, I just need to know where to start!

Any assistance is GREATLY appreciated! Thanks in advance!

EDIT: It seems like this thread might be the fix but it doesn't say anything about the symptoms that led up to this maintenance being required so any clarification is appreciated!

Last edited by holycrapitsjer; 04-01-2014 at 05:50 PM.
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Old 04-01-2014, 05:13 PM   #2
TheDeadner
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Have you checked to see if your battery is loose and moving around when you hit the brakes?
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Old 04-01-2014, 05:19 PM   #3
holycrapitsjer
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Judging by the other symptoms I'm experiencing, I'm almost certain this is mechanical and not something loose moving around in the rear of the car... I just went ahead and checked the battery anyways though and it's strapped down tight! I appreciate the thought!

Last edited by holycrapitsjer; 04-01-2014 at 05:19 PM.
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Old 04-01-2014, 07:27 PM   #4
323in410
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Something is loose or broken. Considering it's with your braking system it's an issue you want to fix ASAP. If you know what you are doing/ looking for get under and check it out. Otherwise get it to a mechanic ASAP.
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Old 04-01-2014, 07:43 PM   #5
wapner
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Longshot: Not on my E46, but on my old E39(They have similar brake pad system) I had a similar noise however I heard it over bumps(like speed bumps) when brakes were not applied, as well as what your describing. If I was very light on pedal it wouldn't happen, but hard to normal braking caused the clank, metal on metal clank sound. It turned out to be one of the prongs that hold rear brake pad(there's 3 of them) had broken off and pad was allowed to flop around, and slap when brake was applied with moderate pressure. New set of pads and spreading the prongs out a bit for snug fit made it go away.
I cant recall noticing anything on the mileage gauge though. It drove me nuts for weeks until I figured it out.

Again, longshot, but that unexplained metal on metal clank rang a bell with me.
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Old 04-01-2014, 09:52 PM   #6
My325XI
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Also check your rear shock mounts and springs.
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Old 04-01-2014, 10:51 PM   #7
TheMagicalHindu
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Brake pad more worn on the rear left could cause this without your brake pad light going off because there is only a rear right sensor...

But if id have to put money on it id say one of your rear calipers needs to be replaced.

Can you record the noise happening?
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Old 04-01-2014, 11:11 PM   #8
seigi009
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OP: This happens to me exactly and has been for the past 2 years and 78k miles I've put on it since. My car runs strong too. Try a search on the boards cause what I found out is there is a software update that can fix it (it's not the brakes). You'll have to take it to the dealership to update it though. I haven''t done it yet cause I don't thing that the thunk is necessarily harmful to the car and whenever I get the money together to go do it (about $150) something else on the car needs my attention. BTW, I have a 2003 325i auto with 153k.

Last edited by seigi009; 04-01-2014 at 11:12 PM.
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Old 04-01-2014, 11:20 PM   #9
mack89
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Rear differential carrier mount, have you checked its condition?
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