http://www.fairlyconfusing.net/docs/zx-transmission.mp3
It is free to grab and use in your music, right here, 48kHz, 16 bit, mono WAV file. Take it, if you like.
If you'd like to make such noises yourself, it's really quite simple. Here's the procedure I used. First I created some data, I used Photoshop to make a picture, I made it 256x192 pixels, which is ZX Spectrum native resolution, filled it with geometrical figures in different shades of gray. Converting the picture to bitmap made gray areas fill with repeating patterns (which make more interesting sounds than just random noise). Then I saved the picture to BMP format, but anything without compression would do. Actually you can use any data file, just (for this example) make it no longer than 6KB (bigger files will crash emulated computer when loaded, where I intend to load it).
I have a Spectrum in a dusty box, but it is way easier to use an emulator. I chose ZX Spin, as it lets you load any data file right into emulated memory and it saves WAV files out of the box. The home page seems to be down, but you can find the program here. Spectrum ROM have been allowed for free distribution, so you can use an emulator, even if you don't own the actual machine.
I decided to use video memory region, so I could see how picture loading works. First I wrote a command to save the region. To skip the trouble of learning how, you can use Z80 snapshot included in the download, just load the file into the emulator. You should see a message, "Start tape, then press any key". Now load the data, using menu "file/load binary file". Pick saved picture file and enter 16384 as the address. Now the screen should be filled with scrambled picture from input file. Pick menu "recording/audio/start recording", enter file name to save to and press any key to start the transmission. You should see moving stripes and hear transmission noises. When done pick "stop recording". This will create fun, modem like noise transmission. Try to pitch it down for some extra flavor. Have fun.
