If I've got the gist of this right, as I didn't see the first post - it could depend on whether the slats are metal or wooden? Wooden ones tend to "give" under weight, and as you move around during the night, the shift in position could be causing strain on the wood, or it just creaks when pressure's lifted from them. Also, if the bed's on castors, movement could cause the same effect. Other than that, it has to be the mattress itself. Even a new one can start to creak if it's not very deep, or not sprung like Miracoil.