It's all down to what sort of ETH supply is required. Amtrak had the same issue with their new GM F40s in the late 1970s. If the train supply was required was AC, in order to get the current at the desired frequency the alternator had to run at the rpm to do that, so for standard USA 60Hz frequency it had to run at 60x60=3,600 rpm. For Europe at 50Hz it had to run at 50x60. The alternator is geared to the engine, something like 4:1, so the main diesel didn't have to run at 3,600 rpm, but it did have to run at something like 900 rpm full speed, hence the noise. AC voltage is by-the-by, it can be transformed as required, the key thing is frequency. Incidentally, the rise and fall in tone of the loco when standing is more noise from the air compressor and radiator fans than from the diesel engine itself, which maintains its rpm well.
Amtrak eventually fixed it by installing auxiliary small diesels in the F40 loco carbody, specifically to provide ETH, so the main diesel just idles when stationary. They seemed to have better luck with these than what the Scottish Region did with the Class 27x2 push-pull on Glasgow-Edinburgh in the 1970s, when they installed auxiliary Deutz diesels for this, which brought a whole range of unanticipated problems.
