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Would the cable length affect the tuning or is 125MHz specific to just the switching on the silicon?

I think what happens is the transformer inside the rj45 connector couples a litte of the differential mode signal (that should not radiate significantly) to all wires in the cable harness (common mode), which will radiate. You can select the cable length so it resonates well, acting like a good antenna, but bad antennas radiate as well (worse efficiency). A few mV of noise is quite well detectable from 10m of distance.

There’s a transformer inside the rj45 connector?

In the device, often integrated into the socket.

Thanks for that explanation, makes sense.

zsellera covered the mechanism of transmission very well. The 125MHz frequency is specifically coming from the Ethernet standard, as its essentially the operating frequency of each pair in typical hundred megabit and gigabit Ethernet. 10Mbps Ethernet operates at 10MHz.

By flipping the port speed between 100 and 10Mbps they are essentially toggling the 125MHz oscillator. I don't think anything is intentially generating the frequency of the tone you hear, it's just intermodulation products of the noise and the LO in the receiver.

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