What's Going On Down There

This is just one of the strange incidents that happened in the old white house on Oglethorpe Avenue during the two years I lived there.
It was the sort of place that lent itself to these kinds of things. Judging from its large, rambling floor plan, it was built around the turn of the century as a home for an old Athens family of considerable means and expanded upon over the years. I know precious little about it's history other than the fact that it changed hands several times before ultimately being split up into apartments about 20 years ago.
I had what was once the atrium on the top floor. Because of this splitting up... the house had some really unusual features. At least, the tenants always assumed they were because of the renovation.
Windows in one apartment were so tall that they literally extended up into the apartment above it, leaving the upstairs unit with tiny foot-tall windows along one wall. A curious half-width door on the top floor led to a tiny, pitch black hallway. At the end of the hallway was an old staircase that went down half a dozen steps before terminating in a 20 foot vertical drop into the cellar below the house. I had a trap door in the ceiling of my closet that led up to an enclosed crawlspace just big enough to hide a person in. My favorite features, though, were the bricked up doors. Most of them served as barriers between apartments... but there was one mysterious interior doorway in my neighbor's place that appeared to be a bricked up closet or bathroom. We were never sure about why it was there.
Anyway, this particular story takes place in the dead of winter.

It was the end of January, and I was alone in my apartment at the time... listening to some music and just chilling out. At about 10:00 at night, there was a urgent knocking on my door. So much so, that the knob fell off on the inside. When I finally got the door open, it was Julie, who lived on the other end of the top floor. She looked a little freaked out. She said that she heard two dogs fighting downstairs and it startled her. Now her downstairs neighbor was a vet student with a gigantic rottweiler mix. I knew it was the only dog he owned and that no one else in the house at the time had a dog... so I thought that someone may have been bitten or something. I didn't hear any commotion from my house, though. We stood in the hall, waiting to see if we heard anything else, when this really loud PING-PING-PING-PING started up. It was loud enough to make us jump and sounded like someone was hitting a copper pipe with a wrench repeatedly somewhere deep inside the house. It would go on for five or six times, stop for a few seconds, then start back up again. It was really loud and echoed off the walls. We hurried downstairs and around to the front porch, where the doors to the three first floor units were. The vet student's house was on the left, a young married couple (who were on vacation) lived on the right. The central unit had been unoccupied since the last tenant (a rather reclusive graduate student from UGA with an expensive-looking car) broke her lease unexpectedly and moved out over Christmas Break without telling anybody.
The PING-PING-PING seemed to be coming from the central apartment, just on the other side of the door. Although there was a light on inside his apartment, the vet student didn't answer his door when we knocked on it. Of the fighting dogs, there was no sign. Anyway, Julie thought we should call the landlord (a nice hippy-dippy type who lived a few blocks down the street) and tell him about the noises. I agreed. She called him and he said he'd be down in a few minutes to check it out.
With him on the way, I got a little bolder. I eased up to the (supposedly) vacant apartment and opened the screen door slowly. With it propped up against my back, I leaned up to the door to see if I could hear anything else beside the on-again off-again PING-PING-PING. At that very moment, my other neighbor (a nice, older lady named Miranda who was a professor at UGA) appeared on the front porch in her bathrobe. She told me "I don't think you should go in there" in a way that instantly made me back off from the door, even though I had no intention of going inside. I assumed that she might know what was going on here. When my other neighbor asked why, Miranda said: "I know this is going to sound crazy to you, but I had a dream about this. Months ago. We were out here on the porch, at night. I remember you could see our breath steaming in the cold air. And I had this powerful, negative feeling..." "About what?" I asked. "About opening the door to that girl's house." she said. Julie's eyes went wide at that, "Oh man." The PING-PING-PING-PING started again. Then it stopped. For good. My eyes went wide at that. In less than a minute, the landlord rolled up in his old beater truck and walked up to the porch. We told him about the sounds we heard. He didn't seem too concerned about them. He said that he had shown the house to someone earlier that day and figured that they may have accidentally flipped a switch that turned on a ceiling fan or something. Julie asked if someone could've broken in and the landlord said no, he always locks a unit up behind himself when he leaves. So he went up to the door and turned the key in the lock. The door refused to give. He turned the key the other way, and it creaked open on its hinges. You see... it was never locked to begin with.
The landlord led us inside with a flashlight. He walked around the apartment with us, careful to check in every closet and in every large cabinet, but found nothing. There was no sign of damage or trespass, and no clue as to what had made the noises we all heard. After five minutes in the apartment, all three of us walked out to the porch. The vet student's SUV was right out front with the lights still on. He rolled down his window and asked us what was going on. He said that he just finished driving home from visiting his family in Alabama. ...His dog was still in the truck with him.