It's likely not very far away. A common problem is a broken ground lead at a transformer. Another is simply a defective transformer. I had an intermittent problem once caused by build-up of lint from a nearby cotton gin. In high humidity, it would create a poorly conducting path to ground. Went away when it dried up and after a rain hard enough to wash it off. It was mostly seasonal at ginning time. The defective transformer was pretty easy to locate, since the beam clearly indicated direction. If there is only one transformer in the area, it's a good suspect. The trick is convincing the power company that it's there. It's not that they're opposed to fixing it. In fact, most will deal with it, because it suggests that the transformer may fail, and a broken ground lead is something they want to fix. Any sort of reasonably accurate direction finding or a clearly stronger signal on a portable when near the pole should be enough to get them to check. It helps if you can be there to demonstrate it for the tech. They may also have one or more means of testing for faults. Some analyze gasses generated in the transformer fluid. Others do other analysis on the fluid or work off features of the current. If the transformer serves only your house, they may even take it out of line to see if the noise stops. No processing is going to do much with that kind of noise, not in any way that you'd want to live with.
In an urban commercial environment, neon sign transformers may be offenders. They just have to be run down by D-F or the portable receiver. If anyone happens to have a well-designed HF receiving loop, it's a great help locating any of this sort of thing.