My fridge/freezer LG Express cool GR-602TVF is making an awful whirring noise, which stops when you open the doors of either the fridge of the freezer.

To add to Mr. Taylor's answer, these circulating fans are usually surrounded by a plastic cover. If not seated on the mount properly, the cover will vibrate whenever the fan is running.

Try pressing on the this cover on all sides to make certain its seated correctly. Also make certain that no food items are touching this cover.

It's also possible that the fan itself is not set or balanced properly and will need to be corrected by your dealer. (Our LG needed a fan adjustment when it was new. Dealer worked on it for 15 seconds and it's been perfectly quiet ever sence.).

Obviously the exact cause of the noise must be located in order to know what needs to be fixed. Fridge/freezer cabinets aren't particularly rigid and so it is very important they stand on a firm surface and they are properly levelled using the adjustable feet and a spirit level. The cabinet should not "rock". The fridge will give a longer more trouble free life when properly levelled and often quieter...

Apart from a condenser fan at the rear (if fitted) which shouldn't run continuously, lots of modern fridges/freezers have internal fans to circulate the air to produce a more uniform temperature throughout. These fan(s) start when the door has been open and then closed and will run only for a relatively short time and due to the cold often damp conditions, they are prone to making strange noises.