I'm going to assume that you're describing the normal sounds of various pieces of the cabin interior rubbing against each other, or expanding/contracting due to thermal changes.
An aircraft's basic structure is a skeletal frame covered with an outer skin. Typically aluminum, but fiberglass-like composites or more advanced materials like carbon-fiber reinforced plastic are also used. All the stuff on the inside, from the seats, overhead bins, cabin walls, galley, even lavatories and so on are simply bolted in place, attached to the floor and other suitable structural members. These objects will all flex slightly on their own when the aircraft is in motion, and you may be hearing creaks in the floorboards or other items. (This is perfectly safe!) Further, because the aircraft will flex, there is spacing between each of these components to allow slight movement. These spaces are stuffed with rubber or foam pieces. It's possible that you're hearing friction between the foam and the objects, but it's probably just all of the items jostling around.
The air conditioning system consists of flexible ducting (including hoses with plastic and metallic film) and these hoses may expand/contract when airflow is changed, or when the temperature changes. So if some crisp outside air was brought in to cool down the cabin, you may hear those ducts crinkle a bit during the transition.
But, like others have said, without hearing the actual sound, this is only a guess.