Yes, many of them can be. Typically the inverter will have a fan that will generate some noise. How much noise the fan on the inverter generates will depend on the size of the fan and its quality (good/bad quality bearings). Essentially, don't expect any inverter to be used in your caravan or motorhome to be silent. Therefore you're going to want to turn it off overnight. Tuning inverters off when not in use is a good idea anyway, as some inverters will drain your battery even when no 230V appliance is in use.