Back in the pre-alsa days when linux used OSS you could pipe /dev/random into /dev/dsp and get noise, you could pipe anything into /dev/dsp and generally get some sort of noise. Possibly can still do this on the BSDs since they still use OSS.
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You can still do it with ALSA if it has the OSS compat modules loaded.
I've done this with my setup fairly recently. I don't recall having to do anything special.