You should be hearing brown noise! (Only on Chrome & Firefox)
This type of noise is great for studying or sleeping.
Brownian noise, also known as Brown noise or red noise, is the kind of signal noise produced by Brownian motion. The term "Brown noise" comes not from the color, but after Robert Brown, the discoverer of Brownian motion. The term "red noise" comes from the "white noise"/"white light" analogy; red noise is strong in longer wavelengths, similar to the red end of the visible spectrum. The sound is a low roar resembling a waterfall or heavy rainfall.