I usually install truss rods on my guitars so that they can be accessed through the sound hole. On my Celtic Citterns and Irish Bouzoukis I have to go through the headstock because of my bracing pattern. Some makers put caulk or bathtub silicone inside their channels to limit noise. I hope my instruments last much longer then the life of synthetic bathroom sealants so I use hot hide glue and a fillet of the same timber. This fillet is glued in position and then I plane it flush with the surface of the neck. It's a little more work, but hey,why not? My customers are worth it! I think this is a great solution to the dreaded rattle I hear from many factory manufactured guitars.