If you have room for it, an oil filled heater is going to be the quietest. I have a small trailer & use a Pelonis Disk Furnace. it is a small (5"x6"x7") 1500 watt heater. The advantage over most other electric heaters is in the auto mode you set a dial on the back of the heater for the temperature you want. When it reaches the set temperature, it drops down the wattage & fan speed until you can barely hear it, and the wattage drops to around 400. As the trailer cools, the wattage goes up (as does the fan speed).
Mine is an old version (1989).
While they are still available, I've been told the newer ones are not as good as the old versions.