Take a FFT of the sound. use this software or another like it.
http://www.arachnoid.com/FFTExplorer/index.html use a good sound card if you can afford it.you could be hearing a beat frequency of 19th or 20th harmonic of the 7.8 kHz oled, and the 150kHz switcher
7.8 * 19 = 148.2kHz :-) welcome to the land of harmonics :-) Steve
smime.p7s
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