So, what you guys are actually hearing is a vacuum pump in the EVAP system. After 5 hours, if all the requirements are met, like the temp sensor reading cool enough for long enough. This vacuum pump comes on to test the integrity of the evap system. If things aren't right, then there are specific codes it sets off. The reasoning behind this, is because the the two valves in a modern EVAP system, activate hundreds of times a minute and are prone to failure after a few years. Only now, that system is depended upon for a smooth running engine. When the system is not attended to in modern vehicles, it leads to carbon buildup and expensive engine damage.
Oh, and I guess it might be some sort of heat pump, like someone else listed, but I have yet to encounter this part yet. The electric water pump in Mercedes vehicles, as far as I know, is used to pressurize the cooling system from the reservoir bottle used these days. It's one of the noises you hear when first inserting your key to start.