Found a story about it at:
http://www.ananova.com/news/story/sm_246696.html

Cats purr to help them recover from illness and injury

Scientists say they've discovered that cats purr to help them get better when they're injured.

The researchers at the Fauna Communications Research Institute in North Carolina call the purr a natural healing mechanism.

They say the purr helps their bones and organs to heal and grow.

It works in a similar way to ultrasound on humans.

Exposure to similar sound frequencies are known to improve bone density.

Dr Elizabeth von Muggenthaler, the president of the institute, said: "Old wives' tales usually have a grain of truth behind them and cats do heal very quickly. The healing power of purring seems to explain their 'nine lives'."

She told The Sunday Telegraph: "We are starting to solve a 3,000-year-old mystery as to why cats purr. The next phase will be to explain the mechanics of the process."

Story filed: 15:49 Sunday 18th March 2001