A strange noise coming from below an abandoned warehouse

Her telescopic vision zoomed in towards a warehouse in the distance, pin pointing a strange, sporadic thrumming noise coming from underneath it. It was very faint, and it took all her concentration to be able to hear just an echo of it. It could be some kind of machinery that needed maintenance but she knew what a boiler or heavy machines sounded like from the long time here, and this did not sound at all mechanical in nature. It caught her curiosity, and being the type to try to handle such things herself, she flew straight there.

Within moments, she found herself inside the semi ruined and rusted building. The interior was mostly empty save for a few rusted out cars. With her heat vision, she could not see any traces of anyone recently being here. However, the sound was closer than before, and it came from deeper within, inside what might have been a small corner office.

"Well, let's see whats behind door number one," She muttered to herself with a sigh.

She switched to her X-Ray vision, and just as she suspected, there was a hidden stairway that led down below. With a smirk, she wasted no time to check it out, pushing the door to the office open and effortlessly pushing aside the crates that covered the entrance that looked like a trap door. Gingerly opening it, she made her way down, keeping her heat vision up in the darkness below. For the longest minute, it seemed to keep on going, but she saw a door at the bottom. Oddly enough, she could not X-Ray through it or the walls around it, which aroused her suspicion that something was definitely up to no good here. The sound was coming from the other side, and she could make out clearly now that it was not something a machine would make. A dull, irregular thrum that seemed to reverberate in intervals of several seconds. There were no other sounds that she could pick up that might be there.

"Alright. Cue action music," She muttered quietly again.

Since she could not see behind the door, it was time for the old fashioned way. Reeling her heel back, she plants it hard against the door, sending it flying off its hinges. Calmly stepping inside, she expected some kind of secret cult or another for her to bust, but her eyes widened in shock from what she saw.