Your truss rod is only one way single rod. If it still buzzes when the rod is loose that means the neck has warped upward. This is rare but sometimes it does happen. In this case you want to force it develope a back bow.

Your truss rod is only one way single rod. If it still buzzes when the rod is loose that means the neck has warped upward. This is rare but sometimes it does happen. In this case you want to force it develope a back bow. Here is an idea that I think I would try. Loosen the rod very loose, then loosen the strings very loose. Get something round and padded like maybe a felt paint roller and place it under the strings midway of the neck. Then tighten the strings in small increments each string having about the same tension.

Go through this step several times until the strings on top of the roller presses down tightly enough until you see the neck starting to bend cupping downward below the roller. Leave it that way for maybe a week or so then loosen the strings and take out the roller, then tuner upper and see what happens. Sometimes we have to put a neck heater on the fretboard with pressure to do this.

If all else fails and the wood does not give in to your efforts, then the last but sure way is to take the frets out of the fretboard slots and recut the fretboard back to true flat and re-radius (12" radius). Then put new frets in and dress them down to accurate. I have seen some cases so bad you would need to remove the fretboard itself and shape the neck then glue the fretboard back on. In tghis case you can leave the frets in place and glue the fretboard back onto the neck clamping it in a way that makes the neck bow downward. Then you can use the rod adjustment to vring it back up to level balancing the wood with the string tension until it all stays flat in proper alignment.