When you strike the strings without producing a tone or a pitch you are utilizing a percussive device to make a musical statement. You can use both hands to make percussive sounds on the bass and create rhythm. Strike a note and stop it from ringing with the first finger of your plucking hand. Repeat this and make a clicking sound this time by tapping the string a little harder while stopping the note from ringing. Create a percussive device in this way. Now play a note with an open string and lightly rest your fretting hand against the string to stop it from ringing. Do the same thing a little harder with the fretting hand this time and make a clicking sound while stopping the note from ringing. You now have two percussive devices at your disposal.
Explore the idea of combining both hands to start and stop notes in percussive ways, and in non-percussive and subtle touch oriented ways as well. You can combine percussive sounds with ringing tones to add variation, color and rhythm to your lines.