On a recent long-haul flight on an A330, at various points during the flight a strange crackling-type noise could be heard through the aircraft. It first happened whilst taxiing for takeoff and then subsequently every two hours or so.
If I had to guess, I'd say it sounded like ice-crystals or something being blown down the air-conditioning ducts. I'm no aviation expert (by a long shot) but I've never head this sound before and wondered what it could be?!
I have recently flown on a China Eastern A330-200 from Shanghai to Beijing and that noise was heard around 10.000 feet both on the way up and on the way down. It sounds awful! Everybody was rather worried, including myself who, despite being a frequent flyer, had never heard that noise. At first I thought it was some fatigue issue of the aircraft skin while "inflating" as internal pressure increased. But a few days later I flew on a Cathay Pacific A330-300 from Hong Kong to Jakarta, and after hearing that noise again, I asked a flight attendant who told me it was likely ice crystals in the air conditioning tubes that form when the outside air is very humid. She said she heard it many times and it's considered normal on A330's.
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.
