Oh yeah! Noise is very important to the sound of a lot of modulation effects, too. Like a phase shifter or a flanger even at pretty extreme settings will be pretty boring on most pure clean sources. The more complex the input, the more frequencies it has to mess with, and the more interesting the output. Most instruments aren't actually all that complex - mostly just the fundamental and a harmonic or two for the sustain portion. In order to get that really dramatic sweeping sound, you really have to either distort the hell out of it or add noise.