Happened to me before, I thought it was a generator running at a construction site down the street. It wasn't that long ago but I don't recall exactly when it stopped or if anything correlated.
I used to think that the generators were running at the bank call center 1/4 mile away. I only heard it when it was very quiet, any noise at all and the low droning disappeared.
I took more vitamins during the early days of the pandemic and magically the generators stopped running.
