You're thinking of the brown NOTE, which has not been proven to actually exist.
It goes something like this:
White noise is a mixture of all frequencies audible to humans, which sounds quite high pitched. It's excellent for focus, against tinnitus and just for it not to be too quiet. Helps a lot of people sleep too.
Pink noise is the same thing except tweaked to sound lower yet still "crisp". It's been shown to generally be even more effective for falling asleep than white noise.
Brown noise is the same deal yet deeper still. It's been shown to be effective against anxiety and, at least in my own anecdotal experience, against anxiety-induced insomnia.
On top of all these benefits, a combination of all three is what headphones and earbuds use for active noise canceling, an effect you can also achieve to some degree yourself.
It's a lower frequency noise, some people enjoy it more. As a bad analogy, it's a bit more like hearing a "waterfall" (brown noise) than "wind" (white noise)
I for one prefer brown noise. Could be a result of me working on a ship for a long while. I always found the deep engine hum with the sea against my porthole to be very soothing. Plus, the rocking motion didn't exactly hurt either.
How battery intensive is it? I tried sleeping to a brown noise audio file in a normal music player app (Music Player Go) and my phone ran out of battery before my alarm went off, causing me to oversleep 😬
If you're the kind of person that plays on their phone to fall asleep, get a 10' cable so it can be plugged in while you're doing whatever you do before sleeping.
Does it have creamed spinach noise? ("Creamed spinach" refers to the color used in the DMG Game Boy's screen, as Sega referred to it in one of its Game Gear commercials)
This app was fully offline previously. Now with premium thing they removed it and i cannot turn of my mobile data to sleep. So this ruined the whole experience for me.