[RFI] receiver noise FT-1000MP

[Kenneth Sobel]

I have an s7 buzz noise on all frequencies 14Mhz-30Mhz. I don't have antennas for the lower bands.
The noise begins at random times and then leasts for hours or days before stoping for several days.
The noise always sounds the same. But sometines the noise blanker can reduce it to S1 and other times
the noise blanker has little or no effect. The noise stops if I disconnect the coax going to the antenna which
is a Force12 5BA. I thought that maybe it was cable interference. So I disconnected all cables from the cable 
modem and disconected the power supply from AC outlet. This had no effect. So I disconnected the cable 
where it comes to the splitter from the pole. This had no effect. Anyway the modem and computer are not in the ham shack.
Does anyone know what could be causing this noise?
Does the broadband nature of the noise indicate power line arcing? Or does the nature of the noise lasting hours or a day and
then stopping for several days indicate that a neighbor is using a device that is radiating rf? 
All morning today there was no noise until 12:30 pm when an S9 noise began. The noise blanker can reduce it to S5.