Hi enov, welcome to the forum. is there any way you can get the history of the bldg, age, any past occurrences. area its in. or to find out if any past occupants experienced anything?.... first i would say to have the electrical system checked. lights flickering, popping sounds, and black smoke are not good...lol.... for safety reasons that should be the first play.... as far as paranormal activity, once everything else has been ruled out, document everything with photos if possible. writing on wall, hand prints etc... if possible set up a camera in rooms with heavy activity. apartments and large bldg's can be tricky, sound travels, as do vibrations. someone running, playing loud music, anything vibrational can travel through the bldg. to include the plumbing, especially if someone has a washer or dryer in their unit..... the voices and the apparitions need to be documented if possible, recorded. photographed etc... (that can be hard to do)....whatever you do, dont try to make contact, dont try to communicate, dont acknowledge it in any way except for or by a prayer of protection to let it know it is not welcome there.