What is an ERG valve?

My "check engine soon" light came on yesterday and soon after that I took it to the nearest Midas to have the diagnostic test run on it - they stated I needed an ERG valve. I have no clue what it is and was told that it could run $160. - $300. for my 1900 Astro van. what is an ERG valve? I have noticed that my van has a low humming sound when I'm driving it.