Hey guys, i was reversing my car today, and all of a sudden i hear a thumping noise. This isn't the first time i've heard this. It sounds like metal expanding and bending (could this be the fuel fumes pressure in the fuel tank?).

it happend twice today in warm conditions and my fuels about 1 month old and only about 8 litres left in there.

EDIT: got under the car one day and notice the muffler was expanding and contracting as it heated/cooled. No idea why this happends but its bloody annoying