It's all done through the magic of a free circuit simulator called LTSpice. Now I know you're probably saying "Gee willikers, Tyler, I don't know anything about running a circuit simulator- that sounds HARD!". Don't worry, Bunky! It's easy and I'll give you a few templates to start with and modify to make whatever weird noises you want.
Not sure it's worth the effort? Here's a link to a ready to play sound file (it is made from "composition_1.asc" in step 7 of this 'ible) that you can try out. I converted it from .wav to mp3 to reduce the download time. http://www.rehorst.com/mrehorst/instructables/composition_1.mp3
There's some low bass in it the sound so listen with headphones or good speakers.
If you like what you see, vote for me!
Note: I have attached schematic files for LTSpice that you can run on your computer, but for some reason when you try to download them the names and extensions get changed. The contents of the files look OK, so after downloading the files just change the names and extensions and they should work. The correct names and extensions are shown on the icons you click to download.