Wedging the old neighbor under the piano may deaden the sound of the neighbor, making life with a new neighbor replacing him more peaceful.
Seriously, it's likely that the neighbor is hearing either your pedaling or the sound of your key strikes. I practice with headphones on my Roland keyboard late at night after the kids have gone to bed, and my son can still hear me from his room upstairs. The keyboard is in the carpeted livingroom on the ground floor of a house with a solid slab foundation, so it's definitely not from vibration conduction. The slope of the vaulted ceiling and the thinness of his bedroom door at the top of the stairs don't help, though, and probably exacerbate the problem.
The sound is much like very heavy raindrops hitting the roof and can be bothersome.