I built this noise generator some years ago, I had to open it up to get the schematic. The reverse biased transistor base-emitter is the noise source. I attenuated the noise because it was used to check the sensitivity of CB radios. If I could hear the increase in noise, the radio had sufficient sensitivity. Almost any NPN transistor will work, the one I used was not marked, so I don't know what is was. 2N3904 should work. If you want the noise to extend to high frequencies, good RF layout is essential. Mine was built on copper clad with glass bead standoffs.