... It is probably a switching power supply switching at 100 kHz.
In that case, you can use a portable AM broadcast receiver tuned
to a multiple of 100 kHz, preferably where there isn't a local station.
You should be able to hear something as you tune through the band
if the interference is strong enough. Then use that as a sniffer and
walk through each room, by any electrical cords, and see if you
find a spot where the interference is strongest. You may have to
tune off frequency when it gets strong if the receiver gets swamped.
Of course, you can do this with any other receiver that covers the
affected frequency range. The AGC in the receiver may make it
harder to detect the peak, but you should still be able to tell
"weak" from "strong".
I find a hand-held 80m DF receiver is convenient for this, even
though the loop antenna doesn't give accurate results in the house.
But not everyone happens to have one of those handy.
A friend of mine ordered a noise sniffer kit from Europe that does
this function in a couple of ways - it provides both AM detection of
the interference, so you can hear what it sounds like, as well as
an audio signal strength function that works on unmodulated
signals. Unfortunately, I don't have the name of the unit handy,
but it is around here somewhere...
It was quite interesting to see which wall warts made the most
noise. I have to unplug the power supply cord from one of my
desk lamps when I'm operating 2m.