Mari,

Have you ever tried one of those noise generators?

They have some that just emit "white noise" that are often used in doctors offices to ensure privacy by blocking sound coming from other rooms.

Then there are other noise generators that play the sound of rain, or ocean waves, or rainforest sounds, or gentle music, etc... I have seen them in the Sharper Image catalog a lot and most of them are reasonably priced (the $40 - $80 range generally).

Fans can produce white noise, so can radios and TV's if you put them on a non-existant station -- but the actual white noise generators tend to do a much better job of actually keeping outside noises out and inside noises in.