Jack when it's that broadbanded it's much more likely to be 2 things; a local source, close by. Anything from a light dimmer, crappy transformer connections, an outside light about to go out, a flourescent light on the fritz...you could probably write a book on the number of things that can cause noise - and another book on how to fix it.
The only way to really be sure is to take a portable with a short antenna and walk around, noting when the noise is weaker/stronger. I would select a noisy VHF air frequency that's not used in our area and start taking notes. Process of elimination works here.
73s and GL..Mike