I am not a luthier and I am sure one will chime in, but I am speaking from the point of view of someone who has owned more than a few old, beat-up, sub $2,000 basses.(Even a few sub $500 basses back in the day.

) My home remedy was the following:
Just to be sure, make sure that there isn't a loose string end vibrating against something. (Its a long shot in your case, but I would check.) I don't know if you are in San Diego or Boston, but especially if you are in San Diego (dry this time of year as you know,) you might try putting a Dampit in the bass. It won't fix the buzz overnight, but one of my old basses used to dry out and develop a buzz and this helped. Check for newly opened cracks or an open seam also. (Take a very thin piece of steel and run it along the seams and see if you find an opening.) All that said, a buzz in an old bass can be hard to track down and you will probably have to go to a luthier. If he can locate the problem easily, he may be able to fix it without opening the bass. (If it is the soundpost, perhaps it has slipped as the bass has expanded or dried out, and he may be able to reset it, shim it, etc. and it should be worth the $) Of course, the soundpost transmits sound within the bass, so I would guess that a buzz may be heard there that originates somewhere else.
Finally, if the bass has to be opened to get to the root of the problem, the value of the instrument will dictate whether that is a worthwhile investment. In my experience with cheaper instruments, it often isn't.
Good luck.
