I have a solenoid valve that is operated by a 230v coil.  It lifts a small armature off a valve seat that allows fluid to be discharged.  The  check valve discharge orifice is 3mm, and the working pressure is 14 barg.  The problem is all of a sudden when the valve is discharging there is a loud buzzing or chattering coming from the valve stem.  It is a balanced system which relys on a small spring force to jkeep the valve sealed under pressure.  It's only when discharging water that you get this buzzing.  With just air it is not aparent.  I have checked mouldings and seat profile and dims are to spec.  It is also so inconsistent, it may happen 5 timnes in a row, then hot happen again.  Could this ba the coil not working in it's sweet spot, or could this be a water hammer thing.  I seem to be going round in circles.
What is the buzz frequency? Is the the same as the electric power frequency 50/60 Hz? If so the problem may be with the holding power of the solenoid. There also should be a thin copper disk in the front of the solenoid pole.
There is a small copper disc aon the inside of the valve where the armature sits.  Not sure of the buzzing frequency..  Is it important to look at the wave form of the coil to determine if it is saturated or not.

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