Originally Posted by kladue
Two possibilities, you have placed an AC coil on a DC setup valve, or the piston that lifts is dirty and not making good contact with the stationary slug nut inside the stem. If it was a dc operated version then you are into a rebuild kit for AC which costs more than the Honeywell valve will, > $100 retail. If not then disassemble valve and clean the top surface of the piston and the inside of the stem, then it should be quiet again. Good luck and do not blow your self up with that setup.
Very solid advise.
DC valves will have a breaker piece to prevent residual magnetism from having the core stick in the core tube.
AC valves use a shading coil to prolong the magnetic field when the current (60Hz) drops through zero.
If the shading coil is dirty the valves can buzz and what you are hearing is the core basically vibrating.
If it is a DC valve with an AC body, there isn't a shading coil, and they will buzz as well.