I’m no expert on this but it’s also one of my bugbears. My understanding is that it’s caused by the relatively high operating frequency of modern PWM controllers which make the motor coils act like loudspeakers. I think the effect is exploited in the more sophisticated units (eg. Shark brushless ESCs) to provide audible programming information in the form of bleeps. I also believe it’s more efficient.

Older ESCs operated at lower frequencies: my early experience was with a home built RCM&E ‘Pompey’ unit (70s) and then an ‘Action’ speed controller (90s). These did not whistle. Action controllers were available until quite recently and were advertised as low frequency units. Unfortunately they no longer seem to be made but frustratingly, the later ones had very limited adjustability regarding neutral position and control range making them unusable with my elderly RC gear. 90s era Fleet controllers which I've also used, don’t whistle either. For low speed scale models I don’t think any slight inefficiency matters. It might be worthwhile investigating vintage units from back in the day.

Currently I use modern ESCs. I posted some time ago about a Veron Titan Tug project and a Graupner Condor 2. I also run a Graupner Elke, a similar model to yours. All use Mtroniks ESCs with brushed direct drive motors. They do whistle slightly, but I don’t find it intrusive. Neither is as loud as your model seems to be (there also appears to be a surprising amount of motor noise together with the ESC whine) indeed the tug is almost silent, even on a calm summer evening on our local lake - its wake is louder.

I’m sure others with vastly more knowledge will be able to offer advice, but I hope my observations help.