alicekuder
I live near the Southwest Community Center (Westwood Village area), so I’m sure I’m not hearing the same hum you are, but I discovered years ago that the huge ventilation fans on the roof of the center create the kind of noise you are describing. About once a year, the maintenance team would go up and clean them and inadvertently changing the direction in which the fans faced. When they face east, there’s not much of a problem because there are fewer homes in that direction, on that level, but when they face west, they really disturb those of us whose homes are directly across the way. I suggest you look for similar circumstances in your area. I.e., commercial buildings with mega-fans on the roof. I hope this helps you locate the source of your hum.