How to create a reversible buzz on a bass?

[Pentabass]

Many instruments make use of a buzz, to enhance the sound. Examples range from coca-cola tops attached to Mbiras from Zimbabwe, all the way to the distortion produced by some tube amps.

I know usually people go on a buzz hunt to eliminate that irritating noise. Based upon those experiences, what would you do if you actually wanted to reproduce it on a double bass?

What mechanical device would produce a reliable, reversible, nice buzz?

thanks for your ideas.

[JtheJazzMan]

Small nuts and bolts inside the bass, something touching the top plate always rattles as well, cellophane in front of the F holes, you name it.

[forester]

cola-caps on the afterlength...
my favorite is thumbtacks on the piano hammers..a-la harpsichord

[bejoyous]

Just take a sheet of loose leaf paper and roll it into a tube. Then weave it through the playing pat of the strings right next to the bridge. Over the E, under the A, over the D, under the G or vice versa.

Or you can open some paperclips a bit and hang them over the thin part of the piece of wood that sticks out into the heart-hole in the middle of your bridge.