Recently I've been hearing a whining noise from the engine bay. After checking, it seems to come from the valve cover. I've checked the motor oil; level seems ok although the oil is black but not smelly. There´s no white foam in the oil either. It is about a year ago (c:a 13000 kilometers) since I changed oil and oil-filter. The car is a Nissan Primera P11 hatchback with CVT automatic gearbox. The whining noise is there all the time, already at idle revs.
It sounds like your engine is oil starved due to how old/dirty your oil is (mileage). The first thing you need to do is get your engine flushed, then quickly followed by an oil change (with filter). 13000km (or ~8000 miles) is quite a bit to put on an oil change/filter. I would bet your owners manual probably says to change it every 5000km. Smaller engines cannot run as long as larger engines, mainly due to the fact they don't carry as much oil in them. I would bet, your engine probably holds about 4 liters of oil (4 quarts, whatever). Newer cars which allow their engines to run 8-10k miles between services have between 8-10 quarts of oil. Hopefully this will help your engine noises; hopefully the lack of engine service has not done your engine permanent damage, but chances are the damage is already done.