Despite being electronic, the Quantum is known for slow feed rates, inconsistency and for jamming. In all honesty, you're probably no worse off with a good gravity loader than you are with the Quantum.
You aren't going to find any "silent" electronic loaders - the very fact that simple electronic loaders act to stir up the balls inside the loader to clear jams is going to create noise.
For that matter, any loader - gravity loaders included - is going to make some noise as you move around, simply because the balls will move around.
Certainly any loader with a sensor will reduce the noise, if only because it isn't stirring the balls around constantly, so that is a step in the right direction.
HALO style loaders - including the Invert Too that iliveforthis99 suggested - are relatively quiet; being belt driven possibly helps in that respect.
Another possibility is the Ricochet "Sniper" range of loaders, that were foam lined to help stop some of the noise. You might find it hard to find one of those though.
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