[Spoilers] What was the noise?

[Arcanist Lupus he/him]

When the guards are looking at David and Megan in the elevator shaft, they hear the tensor:

 

"Huh," the other guard said.  "I do hear a sound, though."

"It's coming from... you know," the first guard said.

"Oh," the other said. "Right."

 

David and Megan then discover the secret room.  But the secret room wasn't making any noise.  So what did the guards think they were hearing?

[Oudeis he/him]

The room wasn't making noise, but the guards knew there was "some secret" behind the wall, just not what it was. So the noise they heard was David and Megan, but anything mysterious in that general area is just ascribed to "The Room" which they don't understand. You and I, the readers, know that it's storage for propaganda and wouldn't make noise, but all the guards know is "if you go in there or even wonder what it's about we will kill you." Anything suspicious that might be related to those rooms is ignored, which includes random sounds in the vicinity (unless they can see something in the elevator shafts).

[Gloom he/him]

The room that David went into had some desks and computers in it. Megan said their were two more rooms like that, but didn't specify exactly what was in those rooms. Could have been server equipment, possibly even a generator. Even failing that, people make noise when they work. Chair legs scraping against the floor, discussions amongst coworkers, random comments, or even levity are frequent distractions that we use to keep ourselves sane when sitting at a desk staring at a screen. So odd noises coming from a top secret hidden facility within a controlled structure were probably something the guards were told not to investigate to deeply.