Quieting cold air return vent
If you shift more airflow upstairs, it's likely it will become noisy. Do not restrict anything unless detailed duct size analysis indicate it is indeed oversized. Insulating ducts will reduce resonant noise, and slightly reduce carried noise such as combustion noise, but it will not stop the whistling and air flow noise at the grille. If you do get insulated ducts, they need to be upsized to make up for the insulation thickness.

To reduce whistling and air flow noise right at the grille, you need a less restrictive grille. Sometimes just a different grille of a different configuration will stop whistling, but not air flow noise. If a less restrictive grille is not possible, the only other solution is to increase the size or add another inlet so there is less demand on the one by the bedrooms.