My Christmas tree is making crackling sounds?

Most likely this is the tree drying out and/or settling. Check the water first. If the water level remains the same, your tree is not taking it in. If your tree did not have a fresh cut along the bottom, it will remain sealed. Trees need to be re-cut before being placed in water. And even then, you have a 1 hour window do to this before the sap seals the base of the tree. You may need to re-cut the bottom or ensure your tree is situated in the water itself. A wet base will keep the tree alive and safe.