This Music Man fretless has a nasty rattle. It rears its head on a number of notes/positions around the board. This particular rattle isn't to do with the string buzzing off the frets (or the board in this case). Here, the rattle is caused by the truss rod itself vibrating.
Inside the neck, there's a channel routed in the wood. The truss rod sits in that channel and, usually all is well. Sometimes, though, there's a small gap—a little play in the tolerances that allows the rod a space to move. That can occasionally lead to a rattling truss rod.
In some cases, simply 'snugging' the truss rod a little more tight is enough to sort things out. Not always, though.
Now, it's worth mentioning that this can be a very tricky problem to resolve. How to proceed is sometimes not clear and, what's discussed here isn't necessarily the solution to all buzzing truss rods. Experience can give you a head-start in making the right call so, if you're in any doubt, seek out some trusted advice.
'Solution'. Get it? I make a glue solution. Get it? Oh, never mind.
In this case, what I'm trying to do is to fill some of the gaps around the truss rod with something other than air. I've watered down some Titebond glue. Watering it down not only makes it flow more easily (there'd be no chance of getting it where I want it at its usual viscosity), but dilutes its strength too—we definitely don't want a glued truss rod.
The diluted glue will, hopefully, flow down the channel and fill some of those spaces. When it cures, it won't be strong enough to actually 'glue' the rod to the neck but will remain in place (albeit probably in a crushed form once the rod is tightened) and continue to fill the gaps.
If this was a fretted instrument, there's a good chance I'd have worked from the fingerboard side—maybe removing a fret or a position marker to drill a hole for access. This would be neater and would let me get closer to the centre of the channel (where I want to be).
Here, though, I've got an unadorned, fretless, fingerboard. Gotta go around the back.
I drilled an access hole at the heel, carefully, until I hit the rod. As an aside, I've spotted that there's already a hole closer to the end of the neck. I chose to ignore it because it meant even farther for my glue to flow and because it was a bit close to the adjustment, 'business end' (no sense risking gumming anything up).
The diluted glue is, slowly, so very slowly, dribbled in. I'm not planning on filling up the entire channel here, I just want to make sure to get a reasonable amount of glue around the rod (primarily at the middle section). I'm obviously holding the neck vertically to allow gravity lend a hand and I've slackened off the rod into a neutral position before starting.
When I think I've got enough, I stop. Then I lay the neck flat on its back. The hope (and when you're working blind like this, there's a good portion of hope involved) is that the glue will pool and cure as planned.
Since the glue in the channel isn't exposed to a lot of air, I play it safe and leave it a few days before working on this again. I want to make sure all of that glue has cured. A rap on the neck seems promising so I reassemble the bass and have a go.
Success. Plug that hole and we're done.
I'm off for a celebratory cuppa. ;-)
NOTE: It's worth stating that, this particular solution can be tricky. You don't want to glue the truss rod into place and you don't want to just inject 'mostly-water'. The consistency of the diluted glue is important. You need a Goldilocks zone—not too thick or too thin. Likewise, if it's not successful first time, earlier glue might block the way for a second attempt. If you're not sure, take the repair to someone who is.