Knocking sound from rear of Mk4 Polo

Hi there,

Wondering whether anyone else has come across this problem.

After my car warms up or if I go on long drives a dull knocking sound starts to appear at the back of the car. At first I thought it was something just left in the boot of the car but I cleared the boot and found it has a very odd pattern of making sounds. The sound comes about a lot quicker when its sunny or warm. It can knock over the smallest of bumps and the longer I drive the louder the knocking becomes. However if I drive the car the day after the sound is not there while the car is cold.

Any suggestions or has anyone had this problem before

Thanks