Diodes can be noisey. I've seen bad switching diodes that were quite noisey while conducting. Zener diodes are the worst, a good Zener does this while conducting. They are used for noise sources at times, such as in the antenna noise bridges.
Resistors are noise generators too, but is calculatable.
Your pre-amp should be a constant current load, so a series resistor could be tried instead of the diodes. This gives a series impedance for additional filter capacitors to work against. Add an array of capacitors to common - large and small film and some r.f. ceramic values. Then add a series r.f. choke (the value appropriate for your frequency) and another r.f. shunt cap. ahead of your final feed choke.
Yes, I think you're right. I'm going with the resistor, caps, etc.
Thanks,
Jon