Tonight I was noodling around on my old bass, a Fender Mustang that I got in 1977 and de-fretted sometime around 1981. It sounded like there was a buzzing coming from inside the neck around the 6th or 7th fret, most pronounced when I played certain notes. I rapped on the neck with my thumb, and can hear a buzzing when I do that too. Could something have worked loose inside the neck? I recently tightened the truss rod slightly, and didn't notice anything wrong at that time. I don't play out on this bass -- mainly I keep it for sentimental value. But I'd still like to keep it in decent playing condition.