Delayed knocking from rear when braking

[Acheron83]

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?

EDIT, please move to the Chassis/Suspension/Braking section.