Buzzing noice form Dimplex FCXA storage heater?

We have several of these heaters. They have a thermostatically controlled fan to help extract heat. On two of them, we get an annoying buzzing noise whenever the fan is switched on. This does not appear to come from the fan itself (which spins OK) but from the area to the right of the fan. Any idea what causes this noise and how to astop it? is there a relay in this area which might cause the buzzing?