This seems a strange omission. No CC or NRPN number for noise level?
There doesn't seem much point - they ignored the last email I sent about the OBX8 bug.
What happens if you run a midi monitor (Midi-Ox or similar) or record midi in your DAW and turn the noise level knob? (Make sure Param Xmit is turned on in globals) The knob should send the same CC/NRPN as the synth recognizes for noise level.