Source of electromagnetic signal in kHz regime
Cold Cathode Fluorescent Lamp (CCFL) inverters are sometimes designed for the 20 to 60 KHz frequency range. The CCFL desk magnifier lamp at my workbench emits EMI at 32 KHz, and shows the same effect on my oscilloscope as described in the question. 

Also, high frequency electronic ballast circuits for domestic fluorescent tubes work in the same frequency range of 20 KHz to 60 KHz.