All materials expand and contract as they become warmer and colder - PVC has a relaticely high coefficient of linear thermal expansion - about 6 or more times that of glass - so you would hear it creaking from time to time as the air temperature changes... My plastic guttering is always creaking...
There's a Wikipedia article here...
http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion
Peccavi
August 2010