Mod Mix in the Controllers section let's you balance the modulation source between OSC3 (however you may have OSC3 set up) and the Noise source. So, for example, you can use the knob to mix ("morph") between a traditional vibrato "woo, woo, woo" controlled by OSC3 to a scribbled up scrabbly noise using the Noise as a modulation source, or any amount between the two.