A quieter filler than polystyrene is definitely possible.

From the shared experience here, shredded cork worked best overall: it was light and quiet, with decent support, though slightly less stable than polystyrene. Shredded rubber was also quiet and stable, but the downside was weight.

Other quiet options mentioned include memory-foam pillow filling, cotton waste, stuffing from a pillow or stuffed toy, and shredded fabric mixed with polystyrene to reduce rustling. These should be quieter, but may vary in how firmly they support the camera.

Rice is quieter than polystyrene but still makes some noise and adds weight. Flour can work and is very quiet, but only if sealed inside a watertight inner bag; otherwise it’s messy and moisture-sensitive.

If you want to keep some of the feel of polystyrene, a practical compromise is to put the beads into small cloth pouches inside the bean bag, which can cut down on rustling.

For the best balance of low noise and low weight, shredded cork sounds like the strongest option from these answers.