The people upstairs are repeatedly making a sound that vibrates off my steel window sill and makes a loud popping/cracking sound in my apartment. Please help me. I am at my wit's end to figure out what this is. Nobody seems to be sitting directly by the window above when the sound occurs below, but I know it is them because they often get up and move into another room after doing it and getting no reaction. What could this be and how can I stop the sound from reverberating into my unit? I would appreciate your expertise.

Have you asked them about it, or asked if you can look at the room above yours?
It could be any number of items that set up a resonance through vibrations - AC, something with a fan, such as a space heater.

Have the landlord take a look .