Most switching supplies are not very stable on their oscillation frequency (free running oscillators) and can wander around in frequency depending upon load, temperature, etc... I would be really surprised if a manufacturer made one run right at 100 kHz. It sounds like a crystal derived oscillator noise (computer, radio, clock, etc...).
I have used a portable AM radio as a bug hunter around the house. If you can pick up the 100 kHz source by tuning around in the AM band it may help you find the offending device.
A neat trick I picked up a few decades ago was to take an old cassette tape deck head and wire it to an op-amp (LM324) with a fairly high amp gain set by a feedback resistor and a 9 volt battery. Using an earpiece I could find all sorts of lower frequency offenders and even trace them down to the component level or wire by moving around the tape deck head. You could do this with parts in "ye junque box" in 10 minutes and have a nifty little toy.
Tisha Hayes, AA4HA