Problems with Switching a Solenoid Valve with a Relay
First -- your snubber network should be across the contacts, AND C5 needs to be changed for a 250VAC X2 capacitor to keep it from being toasted by surges and other junk on the mains.

Beyond that, if you can't stand the noise, replace K1 with a suitably rated SSR but keep the snubber network in place.