help - noises from concrete slab roof

Hi, I have just bought a unit on the top floor of a 4 storey block of units. The units (us because we are on the top floor) has a concrete slab roof. Almost every night we are awoken by noise coming from the roof. It sounds like something is moving in the roof and causing a wood on wood or wood on steel creaking noise. It also seems like that it might be over the supporting walls. We would like advice or any comment please, is this normal, should we be worried, can it be fixed Some "experts" that we have spoken to say it is quite normal for a concrete roof to expand or contract, and this will cause a noise. We would appreciate any comments please Thanks Chris