Countdown to Death

I am a writer living alone on a high floor of an apartment building. Late last night, I was typing at my computer when I suddenly heard three heavy, slow knocks: "Dong, dong, dong". I looked out through my security peephole; the corridor motion sensor light was on, but there was no one outside the door. I thought it was a prank, so I turned back and continued working. Five minutes later, the knocking sounded again, three knocks just like before. I pulled the door open sharply, but the corridor was still completely empty. Only the sensor light dimmed and went out slowly after a few seconds. I felt a chill down my spine, so I closed the door tightly and locked all three deadbolts. Then I grabbed the headphones on my desk and put them on to listen to music to calm my nerves. But the moment I put on the headphones, my face turned pale. I scrambled desperately towards the balcony and jumped off the high-rise building. What happened?