Knocking on sudden braking

Ronan703 · 03-19-2021, 08:33 AM

Bear with me here... I am scheduled to bring my 330i into the dealership this afternoon to look at a rear knocking sound when coming to an abrupt stop. The best way to describe it is it sounds like there is something loose rolling around in the trunk (there isn't) and my worst fear led me to believe the exhaust manifold is loose or something up with the rear suspension.

At the beginning of the week, I was able to replicate it every time, but fast forward 5 days, I can't hear it at all! The only thing I can think of is that my gas tank is close to empty... There's no chance in hell I was hearing the gasoline slosh around on a full tank... RIGHT?! Am I that stupid? I've never heard this is any of my previous cars, granted I am a new to the BMW family.

TheDudeMan · 03-19-2021, 10:19 PM

Quote:

Originally Posted by Ronan703

Bear with me here... I am scheduled to bring my 330i into the dealership this afternoon to look at a rear knocking sound when coming to an abrupt stop. The best way to describe it is it sounds like there is something loose rolling around in the trunk (there isn't) and my worst fear led me to believe the exhaust manifold is loose or something up with the rear suspension.

At the beginning of the week, I was able to replicate it every time, but fast forward 5 days, I can't hear it at all! The only thing I can think of is that my gas tank is close to empty... There's no chance in hell I was hearing the gasoline slosh around on a full tank... RIGHT?! Am I that stupid? I've never heard this is any of my previous cars, granted I am a new to the BMW family.

Sounds like the fuel level.

Ronan703 · 03-20-2021, 08:16 AM

Quote:

Originally Posted by TheDudeMan

Sounds like the fuel level.

Guess I'm just crazy then. I took it into the dealer and they said it was completely normal. To me it sounds like something hitting the undercarriage of the car, but I'll accept being wrong!

TheDudeMan · 03-20-2021, 08:40 PM

Quote:

Originally Posted by Ronan703

Guess I'm just crazy then. I took it into the dealer and they said it was completely normal. To me it sounds like something hitting the undercarriage of the car, but I'll accept being wrong!

I've noticed it too. It's he brakes, they are a little graby.

Ronan703 · 03-21-2021, 06:58 PM

Man, I still can't shake that this sound is "normal". Anyone had any issues with a loose fuel tank or something? There seems to be a correlation with the fuel level (when the tank is empty or full, little to no knock. When half-full or a third, it's pretty obvious). To me, it doesn't sound like a normal sound from a brand new vehicle, but the dealer was adamant that it was a normal sound and they even made me seem like I was being paranoid. It's starting to really frustrate me. I can't zone it out.

Ronan703 · 03-21-2021, 07:06 PM

Quote:

Originally Posted by TheDudeMan

I've noticed it too. It's he brakes, they are a little graby.

Definitely grabby brakes. This sound happens when coming to a complete standstill. So I'll go up to say 30-40, hit the brakes moderately hard, hear all those normal momentum/brake grabbing sounds and THEN once the car lurches back I hear a knocking. Sounds like something swinging in the undercarriage or something loose in the trunk. Exhaust and battery in the back seem secure and I see nothing else that looks off. Handling is fine so I don't think suspension is the issue either. I can't replicate it by going over a speedbump and I don't hear the knocking in any other situations. So strange.

jbarba · 03-22-2021, 01:26 PM

Quote:

Originally Posted by Ronan703

Quote:

Originally Posted by TheDudeMan

I've noticed it too. It's he brakes, they are a little graby.

Definitely grabby brakes. This sound happens when coming to a complete standstill. So I'll go up to say 30-40, hit the brakes moderately hard, hear all those normal momentum/brake grabbing sounds and THEN once the car lurches back I hear a knocking. Sounds like something swinging in the undercarriage or something loose in the trunk. Exhaust and battery in the back seem secure and I see nothing else that looks off. Handling is fine so I don't think suspension is the issue either. I can't replicate it by going over a speedbump and I don't hear the knocking in any other situations. So strange.

maybe is the toll adapter? in my g21 was not properly attached to its place, hidden on the left sode of trunk, behind a removable panel. Sound when brakibg hard.

Ronan703 · 03-22-2021, 02:14 PM

Quote:

Originally Posted by jbarba

maybe is the toll adapter? in my g21 was not properly attached to its place, hidden on the left sode of trunk, behind a removable panel. Sound when brakibg hard.

Good shout, will check that now.

Ronan703 · 03-22-2021, 02:52 PM

I wasn't happy with my dealer's assessment, so I took it to another BMW dealership. Their service department agreed that the sound was out of the ordinary and took a deeper look for a couple of hours. Just received a call that the sound is coming from a "the rear suspension bushing compressing"... And that's it. According to them nothing appears to be damaged or broken and therefore not covered by warranty. That's not good enough IMO. I'll be picking up my car shortly and will have another discussion with them F2F.

03-19-2021, 08:33 AM	#1
Ronan703 Private First Class 72 Rep 117 Posts Drives: M340i Join Date: Mar 2021 Location: DC iTrader: (0)	Knocking on sudden braking Bear with me here... I am scheduled to bring my 330i into the dealership this afternoon to look at a rear knocking sound when coming to an abrupt stop. The best way to describe it is it sounds like there is something loose rolling around in the trunk (there isn't) and my worst fear led me to believe the exhaust manifold is loose or something up with the rear suspension. At the beginning of the week, I was able to replicate it every time, but fast forward 5 days, I can't hear it at all! The only thing I can think of is that my gas tank is close to empty... There's no chance in hell I was hearing the gasoline slosh around on a full tank... RIGHT?! Am I that stupid? I've never heard this is any of my previous cars, granted I am a new to the BMW family.
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03-21-2021, 06:58 PM	#5
Ronan703 Private First Class 72 Rep 117 Posts Drives: M340i Join Date: Mar 2021 Location: DC iTrader: (0)	Man, I still can't shake that this sound is "normal". Anyone had any issues with a loose fuel tank or something? There seems to be a correlation with the fuel level (when the tank is empty or full, little to no knock. When half-full or a third, it's pretty obvious). To me, it doesn't sound like a normal sound from a brand new vehicle, but the dealer was adamant that it was a normal sound and they even made me seem like I was being paranoid. It's starting to really frustrate me. I can't zone it out.
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03-22-2021, 02:52 PM	#9
Ronan703 Private First Class 72 Rep 117 Posts Drives: M340i Join Date: Mar 2021 Location: DC iTrader: (0)	I wasn't happy with my dealer's assessment, so I took it to another BMW dealership. Their service department agreed that the sound was out of the ordinary and took a deeper look for a couple of hours. Just received a call that the sound is coming from a "the rear suspension bushing compressing"... And that's it. According to them nothing appears to be damaged or broken and therefore not covered by warranty. That's not good enough IMO. I'll be picking up my car shortly and will have another discussion with them F2F.
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