The invention provides an air-conditioner for maintaining a temperature of a conditioned space at or near a set temperature. The air-conditioner comprises at least one rotating or reciprocating element adapted to rotate or reciprocate at a variable frequency. The variable frequency including one or more nuisance frequencies at which vibration or noise within the air-conditioner or the conditioned space causes a nuisance. A processor (30) programmed to control variation of the variable frequency in response to operational requirement. The processor is configured to operate the at least one rotating or reciprocating element so as to substantially exclude operation at the one or more nuisance frequencies.