"It's coming from inside the house" is a famous horror movie phrase from "When a Stranger Calls", signifying a threat is unexpectedly close and internal, not external, meaning an intruder is already within your safe space; it's used idiomatically to describe problems originating from within a group or system, not outside forces, though the original horror context is a babysitter terrorized by a killer making calls from inside the home. 

Horror movie origin

The Scene: In the 1979 film (and its remake), a babysitter receives threatening calls, and when the police trace them, they tell her, "We've traced the call. It's coming from inside the house!".

The Threat: The killer is revealed to have been inside the house with the children all along, turning a place of supposed safety into a source of terror. 

Idiomatic meaning

Internal Problem: The phrase means the source of trouble or danger is internal, not an outside influence. 

Examples: A failing company's problem comes from its own management (not competition), or a friend group's conflict stems from its members, not outsiders.