Strange sound from gas cap area / ontop of right wheelwell

[haz]

aha !!

There are two valves under the right front fender that are the culprit of this noise. they open and close for tank venting and are not shutting off. Replace them and the noise will go away

so now I need to google purge valves , what damage them not working might cause my car -and what they do and how many $$$

thanks!