Thruster roughness frequency

I always figured that the thrust roughness was random but I thought I would do a FFT to see if it actually was.

As it turns out there is allot of thrust variation at 31Hz which is the frequency the solenoid pulses at.  I really wanted to believe that the roughness was just some artifact of the setup or issue with the engine but its clearly not. Highly Sensicial! Makes me wonder how much the roughness can actually be minimised.....