Well, if these noises occur after the TV is switched off and can even be heard as you say at regular intervals then there is nothing really to worry about, as its purely caused through the various different types of plastics used in the construction of the set slightly shrinking at different rates, this can be made worse depending on the tension of the screws holding the sections together. Of course the weather conditions affecting the room temperature also play a part in this, and in my case I had to move my PC's LCD monitor out of the bedroom because of the loud sharp cracks it emitted at times during the early hours of the morning. Another thing that can play a factor is whether the TV is wall mounted or not, as wall mounting places extra strain on the coupling of the rear casing, but there is no hard and fast rule except to say that what you are mentioning is something thats not exactly unheard of, and is more of an annoyance rather than an actual fault.