This could be because one of the pipes at the back of the freezer was moved slightly during handling/delivery and is touching something which vibrates when the motor is running. It might be an idea to just moved the unit around a bit so you can see the back and touch pipes etc and see if the noise stops, also look at the motor (large black metal ball type of thing with pipes coming out) which might have a plastic water evaporator tray clipped to the top of it which might be loose, if it is the type of fridge with a larger flat perforated heat exchanger at the back just check if that is not vibrating.
it is the new type of gas they use now I had the same problem and asked our refrigeration engineer at work who told me under new regulations they have to use this new type of gas in all new freezers