Why not go to the library? Many of them allow food (some even sell it!), and they are perfectly happy for you to stay there all day.
On this topic, there's sometimes a really annoying hum where I live. White noise solves it. In summer, I use a box fan. In winter, I used to use this:
cat /dev/urandom > /dev/dsp
And now use this, because it can be set with lower frequencies: http://freshmeat.net/projects/whitenoise/But the best thing is to take a brisk walk in the fresh air, and notice nature. How fortunate to be prompted to do this beneficial thing! (a convenient rationalization that is also absolute truth.)
Your comment brought one idea, I know there are white/brown noise samples, and nature sounds that make the atmosphere more ambient. Does anyone know sound samples taken in coffee shops?