Humming Noise from Humax PVR 9300?

Have a Humax PVR 93oo and have noticed a humming noise (it is not the machine fan) it sometimes increases in volume when different scenes or ads come on - it is on all channels can sometimes be on a channel one day and not the next - it is like an interference hum but can still be heard when the volume is on mute -this has been worse since the Freeview upgrade