Other comments seem to have covered the advice around checking insulation and system charge, so I’ll say that if you need to get a window unit make sure it’s an inverter unit; they’re typically advertised as ultra-quiet. The reason you want an inverter air conditioner is purely the efficient gains you get over a traditional on/off compressor; they use around half the electricity in some cases for the same cooling.

All of the U-shaped units from various brands use inverter tech, but they might be hard to find because a lot of them are rebranded Mideas. LG’s ThinQ “dual-inverter” units are also very good, I have one that’s served me well for a couple years and it’s very quiet. You can barely hear the compressor start up at all aside from a faint electrical whine for a few seconds and the sound of refrigerant moving through the system. The fan moving the air is the loudest part by far (and it’s not bad).

All of that said, consider replacing your central ducted unit with a multi-head mini-split system in the long run. For residential use they’re typically the most efficient you can get, and you have the bonus of being able to control room temperatures individually. A mini-split system uses inverter tech too just like the good window units. In fact, the U-shaped window units are just miniaturized mini-split systems in a rigid housing.