Search for "true" ambient. With this, i do not mean the popular idea of "ambient" that simply means "atmospheric floaty music". The actual ambient-genre is something very different.... something that is "just atmosphere", with any music (if there even is any) being secondary (if there is any sense of melody, then the purpose of it is not a "song", but rather the feeling and mindstate that it provokes).
Two examples to give you an impression of how actual ambient sounds like:
Steve Roach - Structures From Silence (more melodic and "happy" sounding, yet repetitive as usual for ambient)
Vir Unis - Aeonian Glow (cosmic, floaty, extremely uniform)
There is however one problem with that: You mentioned that you're asking for it to drown out environment noise. True ambient is about the worst style for that, because it tends to be very "uniform"... typically no sudden changes or chaotic transients. Now, environment noise tends to not be uniform, but instead chaotic transients... thus, the environment noise will continue to spike out in that uniform sea of ambient.
On the other hand, if you're wearing halfopen or closed headphones, the environment will probably be dampened enough, that even ambient can drown it.
As for how to drown out env-noise.... headphones, 50% whitenoise equalized to human hearing curve (without eq, it sounds like shit!), 50% sea waves or similiar. The white noise is uniform enough that it will raise your noisefloor, yet you will stop to notice it after a while by getting used to it (in other words, it's a volume knob for your ears). The sea waves in turn add some random yet repetitive transients, to make env-transients more difficult to spot and thus easier to ignore. Because sea waves too are noise, they blend nicely into the white noise and thus themselves do not "stick out". Need an isotank for your ears? This is it. I've done this - you can fall to sleep after 15mins with this stuff.
Downside: It's purely functional. This doesn't "sound nice". Aaaand you'll have to mix it yourself.
