Information
The Supressed MP5-N (Or Noiseless Viper as the game refers to it) is a basically MP5-N with a supressor added to make it very useful for stealth operations, but other than it has no differences and most players will pretty much ignore this weapon, largely because its easier to add a silencer to a MP5-N, and the location of the weapon makes it dangerous to get it.
Location
The weapon can be found at the end of the tunnel, facing the entrance Depot, getting there is often very dangerous due to a large number of Springboard Anomalies, all of which produce no annomalies, at the end of this you'll find a corpse of a Stalker (Likely to be the previous owner of the weapon) near him (or on occasion in his inventory) will be the weapon.