What is this noise on our Rover 25??

[kendunlop]

When I drive more than 4 or 5 m iles or so and then stop, within a few minutes, there is a loud metal sounding "POP" on our Rover 25 (2002 model). At first I put it down to something expanding under heat and then contracting when the engine or exhaust cools down. The noise comes from under the car by the way around the middle. Another idea I had was that the fuel filler cap was blocked and it was the petrol tank, or a baffle which was expanding and contracting under the suction of the fuel pump. I drove several miles with the fuel cap off and the "POP" still came but not as loud. Anybody any ideas as my local garage is stumped. The only way to locate it would be to drive for 5 miles, dirve straight onto the garage ramp and stand and wait. Anybody have any clues??

[Panic Button]

I used to have a Peugeot 309 that did something similar.

I never worked out what it was.

Sorry