Shaking Bed (2)

This is the second time I have submitted a story and of course, it did not happen to me, but to my fiancee when he was little.

I don't remember in which house he was living in, but he had a bedroom which was directly opposite the bathroom and he says that whenever he would pass by his room or when he was in the bathroom out of the corner of his eye he would see something moving in his room. It turned out that his bed would be shaking...not violently like in the movie "The Exorcist", but trembling. Curious he would go into his room to check it out and as soon as he stepped in the room, it would stop. He would check under the bed, next to it, behind and he could find nothing that could cause it to shake. It would always happen when he wasn't in the room, and it wouldn't happen only in the middle of night, but also in broad daylight. To this day he still wonders what caused it.