Yes, I think the problem is related to the power usage of the card, and not a radiated noise. When the card starts reading, there is probably a higher power usage spike, the ssds3 regulators will try to draw more power from the internal console regulator, and if it's not fast reacting, there will be a small dip in the voltage level in the internal console power lines till it reacts. That will probably affect the amplifiers in the audio circuit, or anything that is referenced to that voltage line, causing some alteration of the amplitude, and being heard as a buzzing noise, and it's generally happening at a constant 75Hz frequency (the 1x CD read speed is 75 sectors per second) and it's audible.
The SD card interface in SSDS3 has capacitors to try to avoid these spikes, by providing the neccesary peak current, but they can only react to peaks, if it's sustained, they will eventually let that increased draw pass to the regulators and to the internal console power circuit (yes, we tried with larger, smaller, different combined values, and it didn't affect noise at all).
Probably some SD cards are more power efficient, or have less peak current usage, or maybe the increased power draw has a slower ramp that allows the internal console regulator to compensate that increased current requirement, and doesn't cause the power noise.