Switch EVERYTHING else off one at a time.Interference sources:
Soldering Iron (I scrapped a controller as it was just a Triac based like lighting dimmer switch)
LED, CFL, Florescent lights
Low voltage Halogen (may use 12V SMPSU)
Any light on a dimmer switch
Mobile phone / Tablet / Laptop
LED displays (the multiplexing) like on Alarm clock or old frequency counter
PC PSU, even if "off" unless it has a physical on/off switch.
Fridges, freezer motors when running.
CPU in Dishwasher, Cooker, Microwave even when doing nothing.
TV set sometimes even in standby, a CRT can be better or worse than an LCD.
Set top box. Sky boxes have a fake standby that only kills the modulator and AV on Scart /HDMI as they need to get updates (or content if a plus model).
Anything with a Standby mode rather than a real disconnecting the mains can produce interference.
Use a portable radio on batteries on LW to identify source and verify it's via mains (if it has a mains PSU does it get noisier on mains?). When you put the portable right beside the offending article you will hear the exact same flavour of interference. It may be worst every 45kHz (or some other multiple between 18kHz and 90kHz) etc rather than the same all over the LW.
Extreme test for an AC/DC Radio ... 134 off D Cells