We might need a bit more help - model designation would be useful.
Waeco flog two types of 12/240 fridges, the compressor type and the peltier type.
The compressor type is the same (well almost) as any domestic fridge, pumps refrigerant around the piping, thermostat controls when it turns on and off, has a fan to help cool it down, etc etc.
The peltier type is an electronic heat pump with no moving parts - except a fan which turns on when the cooling cycle is running.
Both types will cycle on/off when the pre set internal temperature of the fridge is reached. Setting a very low temperature means the fridge cycles on/off more frequently to maintain the low temp. Changing the thermostat temp to higher just before bed will reduce the chances of the fridge starting up overnight - but will depend on ambient temperature. If the fridge door stays shut overnight you may not even need it on except in very hot weather or if you have the heater running.
You can stop the noise by turning the fridge off at night, and by freezing a bottle of water/ice pack during the day and letting them keep the fridge cool overnight.
I think all Waeco fridges have to be fitted with an appropriate air gap around them (100mm rings a bell), so just check that someone has not put insulation around the fridge and actually reduced its effectiveness.
I thought these Waeco's where supposed to sort of shut down to an internal