Sound of a vacuum cleaner? (White noise)?

It's probably a constant sound which makes you relax, just like the sound and motion of a car or train, and the sound of rain on the roof. It's also the feeling of security that you experience when, as you say, your mum is hoovering, or your dad is driving. The example I gave about the rain is especially true where I live where most of the roofs are made from corrugated iron, and so qite often the sound of rain is the signal for many families to all be inside and asleep. Funny, but true.