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I’m frustrated

MPC500 noisy inputs/outputs

I was growing tired of my unit producing annoying sounds (computer jittery noises mixed with a pink noise and a constant tone of approx 3kHz). Being an electro engineer, I figured this is most likely a decoupling issue and I decided to take the unit apart and check for bad capacitors.
An ocular inspection electrolyte capacitors gave no such indication, but I inspecting the schematic ( led me to the conclusion that the big, blue Lelon-branded caps were the same used for decoupling the voltage rails.

As I replaced them with fresh capacitors of the same value, the noise floor receded. However, there is still the constant tone and I can't seem to figure out where it originates from. It might be coil whine from the inductors but I simply don't know.

Is this a known problem with these units? How can I fix it?

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