As the 319/769s still rely on opening the window to get ventilation, I can't imagine they will be that quiet inside...
From a passenger point of view, the noise of the 319s vs 150s is subjective, the PMS (which is essentially unchanged on a 769 conversion) the predominant noise is from the transformer on starting, but that is then replaced by the DC motor/pinion whine. No worse than any other MKIII EMU and highly dependent on load. Once the cruising speed is reached, it fades away to little more than general rail noises. The power electronics on the trailer car, (also unchanged in the conversion) only makes it's presence known on initial pulling away and can't really be heard once outside station environs. The diesel/generator sets in the driving cars obviously do make DMU-type noises, but the engines are new and probably much quieter than the Cummins in the 150s, maybe sounding more like a Man 170/172 engine, plus the reduced time that they will be spinning at maximum revolutions.The quietest vehicle will be the ATS, with the only noise on that car itself being the MA set spinning and the main compressor running periodically. The PMS car on 319s is probably noisier than the general ambience on 15x units, especially at speed.
Obviously we will have to wait until they are in service to do the acid test.