Unexplained Static on My Old Radio Receiver

I was fixing up my grandfather's tube radio last weekend — you know those old Philco models that weigh about twenty pounds and glow with this warm orange light? I had it all disassembled, cleaning out decades of dust and spiderwebs, when I heard something. Not the normal AM station fading in and out but a distinct pattern of static, rhythmic almost like someone was tapping on glass somewhere miles away. It stopped as soon as I touched the antenna wire. I've been thinking about this constantly because my grandfather used to tell stories about radio operators during the war who could hear things they shouldn't have heard — phantom calls from stations that didn't exist and numbers being repeated by voices that sounded like recordings of people he knew but was hearing them live in the room with him. He said one time a voice on a dead channel called his name twice before cutting to silence. I can't help wondering if what I heard last week was something similar or just my imagination running wild because of all those stories.