Delayed knocking from rear when braking

[Acheron]

I have a knocking sound from the rear of the car which very audible in the cabin when braking sharply. The sound is delayed by a half-second to a second after braking and sounds like something rolling around in the trunk hitting the passenger seats from the rear.

Here's the kicker: I used to think it was random as it came and went, but recently I managed to track it down to only occurring when the my fuel tank is between 1/4 to 3/4 filled. With a full tank I get no sound and with a tank under a 1/4 the sound disappears again. I can reproduce this at will now.

Anyone got an idea or experienced the same?

 

[S1NGH 911]

It's been a common issue for a flap to cause that noise with the 1 series, BMW haven't found a solution for it yet :thumbdo:

Seen 2 others with the same issue :thumbs:

 

[Ralf*]

don't know if it is similar, but on my E38-740i, I used to get this "thump" from the rear, and it was almost always as soon as I came to a stop at a set of traffic lights.

It physically felt as if someone was walking behind the car and then intentionally hitting my car or kicking my car.

It turned out to be the petrol tank "tin-canning" due to blocked vacumm relief valve, and this has caused the petrol tank to crack, and seep fuel.

 

[Webmaster]

....his has caused the petrol tank to crack, and seep fuel.

Hmm, sounds safe...:fencelook: