I am a complete bell end!
20 days ago my brother came to visit, was out out in the Polo and filled up with V-power, he went in and bought two tins of drink, i never drank it and didn't seem to recall seeing it until now!
Found a tin of this underneath the passenger seat as i lost a packet of chewing gum so put my hand down there to feel for it. Its been there all this time bouncing off the seat runners hence the noise
Yes i did road test it afterwards and the clonk has vanished