I’ve probably fixed well over 20 loose truss rods at Taylor. We used to put BMX bike grips on our 7/16″ socket drivers (for the neck bolts), the soft rubber was great for wrapping against the back of the neck to check for a loose rod. You could use your knuckles, a rubber mallet or felt headed hammer as well.
We would remove 2 fingerboard dots, or other inlays in the 5-7 area of the fingerboard. it’s good to have 2 access points. We would then drill an appropriate size hole down until you hit the rod. You can get “Flowable Silicone For Windshield Repairs” from Amazon, or the auto parts store. We would actually thin it out a little more with Naphtha. Put inside a plastic syringe. While cranking the truss rod nut between totally slack and back-bowed inject the silicone in 1 hole for a bit, it usually will start flowing out the other hole. Let it dry overnight while the rod is totally slack. Put the inlays (usually replace as we busted out the inlays with a spring loaded center punch). You can also go through 2 fret slots if inlays are too difficult. I would say a few hundred rods were fixed over the years and nothing ever came back. Nor did any silicone leach through the Mohagany and discolor the back of the neck. (which I was always wondering about)
Good luck! Let me know if you have any other questions.
Dave Staudte