This is a very old story. I was about 11 or 12, with my older brother and some neighbourhood friends, making compact cassette mix-tapes - remember those? - one evening. For those who remember, it's a painfully worksome and slow process, as you have to copy songs from one cassette to another in real time - listening to one while recording on the other.
The four of us were at it for the better part of the last three hours, it was almost 10pm, and we were on my neighbour's porch, using two double-deck cassette stereo players to copy two cassettes at once. Fancy stuff! Well, once a song was copied, we would listen to the beginning, fast forward to the middle, and then to the end, to be sure we got it all. Once the last bit played on the last song we copied, my brother was about to press the stop button so we could start copying another song when we heard some other sound starting right after the song. We were copying to what we called a virgin cassette, a brand new, never used cassette, so there should be nothing after the last copied song. But there was. We let it play. It started very low, we cranked the volume all the way up, until we could distinguish what we were hearing: a compassed drum-like beat, much like the ones we expected to hear on cults gatherings in the early 90s. It went on and on, keeping the beat, two compassed drummings, one long one short, never stopping, never changing. I'm not sure how long we listened to it, but we realized how mesmerized we all were only when the sound of a car passing on the street startled us all, and my brother finally pressed the stop button.
We looked at each other, no one daring to speak first. My brother ejected the cassette, got up, and started walking home with it in his hands. I took my own cassette - the one we would copy songs to next - and followed him. No good nights, no see you tomorrow. A silent agreement that we were all frightened and that was enough for the evening.
It took us a whole week before we could get back to it one weekend, in plain daylight. We listened to the cassette from the beginning, waiting for the last song before the beats, stopped it right as the song ended. For a millisecond, I believe we all wanted to let it keep playing, see if the beats would still be there. But we just copied the next song, came back to check the copy with eager anticipation. The copied song was there, no beats on the background, to everyone's relief. I don't believe my brother listened to that particular cassette once after we finished it.