This is normal operation. When the 640H is in Standby mode it is in a low power mode but not actually off. This is so the unit can both serve content over the network to other units/computers and perform other tasks (e.g. background compression). When leaving the unit unattended for long periods of time you should switch the unit off fully with the On/Off switch on the rear panel.
The 640H has inside it a powerful processing engine that naturally generates heat. There is a small fan to cool the processor inside the 640H. This fan is designed to run continuously and may be audible. In addition, when the unit is processing data (playback, ripping etc), including when in Standby mode, the hard drive may also be heard operating. When the 640H has finished all it's tasks the hard drive will return to a low power in-active mode.