munster joinery noisy windows

i have grey upvc munstery joinery window that are two years old. whenever the sun comes out, or even when it just brightens up a bit they start to creak and i get ugly snapping sounds too. this continues in the evenings as well when they are cooling down again.

i've had a maintenance guy down down several times, and to be fair he is really helpful, and has reinforced the windows where he can and it has helped but certainly not cured the problem. the noises happen repeatedly for hours on end.

he reckons he has done all he can. days when its dark and grey all day are totally fine, but even a hint of brightness is a disaster. when i have visitors round, they nearly jump out of their seats when the creaking / snapping happens.

i've never heard a noise from my north facing windows, pity my main rooms are south facing.

has anyone else got this issue? and does anyone have a contact at munster for a manager, i can never get passed the lady thats on the service dept phone.