That's great news. Sounds like RonH hit the nail on the head with the twisted cable suggestion.

The zener diode selections seem like good first cuts at a value. I still think you will find that if you can relocate the zener and the standard diode right up next to the solenoid it is meant to protect, you will experience a worthwhile reduction in the noise generated by the switching solenoid to go along with the improvements that you gained from the twisted and shielded control cable.

Do you think this suggested relocation of the flyback protection components will help, RonH?

hgmjr

I'm guessing the solenoid is very rugged, and the MOSFET is what needs to be protected. I would put the zener as close to the MOSFET as possible.
Having said that, I suspect that a 3W zener is going to have relatively high dynamic resistance, and if the solenoid current is on the order of hundreds of milliamps (an amp, maybe?), then the MOSFET may break down if you don't take it into account when selecting the zener. NTE has some humongous zeners (up to 50W).
I simulated a 1 amp, 55V solenoid with 3 different snubber circuits. One just has a diode, one has a diode in series with 150 ohms, and the other has a diode in series with a MOSFET-assisted 150V zener (don't laugh ). I couldn't find a spice model for a high power, high voltage zener, so I didn't run that sim. If the dynamic impedance is on the order of 50-100 ohms, you can do the math. The zener impedance is important if the solenoid current is high, because the peak flyback current is equal to the solenoid current.
The sim uses an odd symbol for the MOSFET, because the model is a subcircuit with 4 pins (D,G,S,B).
Note the damped oscillations in the zener-protected unit. This is because there is no damping after the snubber circuit ceases conducting. The inductor rings with the parasitic capacitance across it.

 

It will definitely be interesting to see how well the zener-diode in series with the standard operates real-world versus the simulation.

hgmjr.