flat roof help

RoadeRunner

Hi all - we have a property with a flat roof, single storey approx 350 sq mtrs. The roof was over 20 years old but sound but with I think its called strammit board insulation. We researched having roof re done with extra insulation and after loads of quotes agreed on having new roof relayed over existing felt roof. New roof was a felt type material by Polyglass. Roof done last November and no problems at all until the weather started to get warmer a couple of months ago and we started hearing strange noises on different parts of the roof area from early evening until early in the morning. Sometimes its really load bangs and crunching sometimes its almost like a running or ripple effect and very frightening. I spoke to the company who did work and they said they had never had any problems before and had not heard of anything like it and would speak to Polyglass. They did not come back to me so chased them up last week and asked them to arrange a meeting and again nothing. I have tried to research this and found a few isolated cases but not how to resolve the problem. Its a big roof and I really don't like the idea that it would have to be removed can anyone help me ?