An article from Inside Catholic below.
Something that's important: if you experience phenomena like this, do not assume that the entity is benevolent, or is even the soul of or the deceased person you assume it is. It could be a demon who is mimicking the behavior of the loved one, knowing full well that the grieving family member(s) want to believe it is the deceased person. "Ah hah! It's knocking the same way, therefore it *has* to be Aunt Shelley." That can be its way in and this is dangerous, and you have no way of knowing. Call a priest and get your home blessed.
Gerard J. M. van den Aardweg
Man's inborn religious instinct tells him that there is more than the sensible world; indeed, it "never feels natural to accept only natural things," as Chesterton said. But about the afterlife we have more than just an intuitive inkling. Throughout history there have been manifestations -- some vague, others more concrete -- to support this ineradicable and universal intuition.
Today, phenomena such as near-death experiences, spiritualist practices, and "channeling" or "phantom-chasing" (attempts to technologically register the presence of ghosts) attract a lot of attention. Of course, false explanations abound, and valid experiences of the paranormal can lead to superstitious beliefs and spiritually dangerous practices. Yet there are also many reliable, critically examined phenomena that are not hoaxes or products of a confused or disturbed fantasy, and neither are they to be dismissed by Christians as occultism. These must be taken seriously.
In the course of my years as a psychotherapist, I have come across several such trustworthy manifestations: people -- who never had such experiences before or after -- claiming to hear the voice of a deceased person, or to see him, briefly but unmistakably. The "seer" typically does not know what to think about it, but is certain it was not a dream. In some cases there are multiple witnesses.
One client related that during family meetings on Sundays after the death of her mother (probably by suicide), precisely at the time her mother, during her life, normally served a drink, there was a loud, urgent rapping on the wall of the room, exactly the rhythmic and impelling way her mother used to firmly rap on the wall when she wanted to call her (adult) children to her room. Initially, no one in the company wanted to admit that this was strange, and they searched the house in vain for natural explanations. It was a recurrent phenomenon, but only during these customary family reunions. At last, one daughter, much excited, impulsively exclaimed, "Mother! Go away!" The rapping abruptly stopped, and was never heard again. I verified the story.
Italian journalist Vittorio Messori, known for his dialogues with Pope John Paul II and then-Cardinal Ratzinger, said that when he was a young agnostic he once experienced an unmistakable signal from the "beyond." Exactly one year after the sudden death of his uncle (by an accident) -- and, as he later found out, at the precise moment in the night he had died -- Messori was awakened out of a sound sleep by the insistent ringing of the telephone. At first, he didn't get out of bed, for the telephone was at the opposite side of the apartment and he wanted to go on sleeping; but as the ringing went on, he reluctantly rose, crossed the apartment, and took up the receiver, now fully awake. He heard the noises and crackling that at the time were typical of long-distance calls, then suddenly a loud, clear voice that couldn't be but the voice of uncle Aldo: "Vittorio, Vittorio! I am Aldo! I'm well! I'm well! (Sto bene!)"
Although the meaning of this unexpected intervention from the afterlife is not certain, the experience cannot simply be dismissed...