From GameWatcher: “The Whistle in the Dark is the latest in the string of weekly new weapons added to Fallout 76. As was the case with its predecessors, obtaining it requires completing a set of five Survival mode challenges.
Here’s everything you need to know about how to get the weekly Whistle in the Dark weapon in Fallout 76.”
Source: N4G PC Fallout 76 Whistle in the Dark Weapon Location