I use sox sometimes:

sox -t sw -r 44100 -c 2 /dev/zero -t alsa pulse synth pinknoise vol -9 dB band -n 800 1200 fade q 5 30:00 10

It's filtered pink noise; last time I checked sox didn't have a brown noise generator.

Edit: what do you know, I just checked the man page again, and sox does now have a synth brownnoise option. You can play with the "band" parameters (or omit it) based on your local noise spectrum.

Edit 2: you can also use the "bass" and "treble" filters instead of "band"; the former are easier to understand IMO. It's been a long time since I read the sox man page; it's crazy how much it can do.

Thanks for the introduction to a curious tool.

I tried your command on OS X but changing alsa to aiff but nothing plays. Any suggestion?

I worked out that command line years ago, and I don't even remember what it all does. And all of my machines are linux, so I can't test out what it is for OS X. But from looking at the soxformat man page, I'm guessing you might try replacing "alsa pulse" with "coreaudio".

You can also look at the output of "sox -h" to see what audio device drivers your sox has.

Thank you! I'd never heard of sox and it looks potentially useful and a lot of fun to play around with.