Probably a power supply. Have you tried disconnecting the antenna from the SDR? Scan the entire HF band and this will tell you if anything is coming through the USB connection from your PC.
If that doesn't help then try connecting a simple whip antenna to the SDR and see if the noise is local. You can also use a shortwave radio and tune the frequency in question. Sweep the house for any obvious signs of RFI.
Turning off devices will not necessarily prove useful. You would have to turn off the breakers. If you can't turn off the breakers, then disconnect any wall-wart power adapters and pull all of the electrical plugs (from devices) in the house.
Good luck.
