Between my house and the shop building stands the base of an old radio aerial. The original had a top mast that was cantilevered and I removed that portion when I built the shop. But I left the base, which is quite solid and anchored in concrete. It's steel, tubular, about seven inches in diameter at the base and tapering to about four inches at the top. It is about 24' high.
Yesterday afternoon as I stood at the kitchen sink I noticed that the top of the mast was gyrating. I'd never seen this before and went out to see what was going on. The top of the mast was moving back and forth in a more or less circular movement that could not be missed.
Wind? That's what I thought, but there was no wind in the space between the house and the shop and the trees were completely still. I went out front and looked at the windmill out in the pasture. Not revolving at all. It was a dead calm.
I shall ponder on this, but right now have no explanation for it.