Billy used two of these noise suppression pedals during the Arising! and Machina tours. Following is some info from the Boss web site:
The NS-2 effectively eliminates signal noise and hum while preserving the tonal characteristics of the original input signal. The natural attack and envelope of the signal itself are unaffected because of BOSS power to other effects units.
An NS-2 can also be seen plugged into a Crate GX-15R practice amp in a studio shot from the Teargarden era.