Regeneration valve is part of the emissions control system. Fuel tank vapors are captured and stored in a charcoal cannister. During regeneration valve operation, vapors from the charcoal cannister are drawn into the intake manifold and burned.

This valve is often referred to as a "purge" valve because it purges the fuel vapors......

You can try to spray some WD40 into the upstream side of the valve while the engine is running and the purge valve is operating--that might quiet it down a bit....

J. M. van Swaay