Very similar to wooden dowel Iea I found that the fiberglass "glow rods" that electricians use to reach through and grab cables work great for this. And they are made up of short pieces that are screwed together so they are as long as you need them to be.
I had exactly this situation that you describe! Sounded like brake pedal would rattle at any slight bump in road, pretty annoying as rattles go frankly.
After investigating I found that the noise was a vibration coming from a black plastic electronic module which is mounted up on the left side...