I have new solid interior stained doors and they all keep making cracking sounds when they are closed and left open, even though no one is touching them. Can you please tell me what it is? I am sound asleep and get woken up by these sounds.
One possibility is a slight cracking of the stain or finish coat on as sections of the doors as they expand and contract based on changes in temperature and/or humidity.
Many doors have inset panels that are not rigidly glued in, but sit in channels in the rails (cross members) and stiles (upright members). When these doors are finished, the stain and finish coat may create a weak bond where these panels are joined. As the doors shirink and swell, the panels shift, breaking these seals. There also may be a slight edge of finish that remains, and as the doors continue to shift, that edge may make noises even after the seal is broken.
One way to reduce dimensional changes is to make sure all six (6) faces of a door are sealed or painted. Often neglected are the top and bottom edges, and moisture can be absorbed through those raw faces increasing dimensional shift.
This is hard to say really but it could be a number of different things.
If the door was left in a cold or damp place for an extended period of time then the suddent adjustment of being brought into a warm dry house when it was installed will cause it to expand to its new environment. This expansion can sometimes result in the door grabbing on the inside of the door frame or in solid wooden doors can result in friction being built up between wooden pieces while expanding and then suddenly they can release and cause a small crack sound like what you are describing. This is typically a temporary thing and will slowly stop happening over time.
Another possibility is that the door may not have been built very well. In a solid wooden door the wooden pieces are typically held together in much the same way cabinet pieces are held together, with biscuit pieces and glue. A liberal amount of wood glue will exist between two pieces and they are usually clamped together until fully dried. If one or more pieces were not cut square and yet were still clamped together with glue, then there could be significant stress pulling on the glue as the two pieces wouldn't naturally be seated next to each other. Over time this stress can cause the glue to start cracking suddenly and the overally structure of the door may be weaker as a result.
A final possibility is that since solid wooden doors are heavy, if they are not secured properly to the door frame then you could be hearing stress on the hinges, hinge screws or hinge screw holes into the frame. If the door was installed into an existing door frame using standard three hole interior door hinges then it may not be held secure enough.