Fridge Freezer very noisy!

Question

I have a Hotpoint F/F 2 months old Model FF 175BP(previously I had a separate Fridge and Freezer which were v. quiet.
This new Fridge Freezer switches on and off continuously EVERY 30 MINUTES and often rattles a lot.
There are two controls - one for Fridge which I have set just under medium and the frisge works well and stays at about 4 or 5 degrees all the time.
The frezer control I have set just below medium setting and freezer works no problem at about minus 8 or 10 on my thermometer.
When the unit cuts in there is initially just a whooshing sound (is this the fan) which is ok. After a few minutes there are other noises like rattles as if a compressor is working.
As i have an open plan kitchen diner I find all this annoying. Should the cut in only happen every few hours not every 30 mins?
Also, when i put say fresh bread and unfrozen items in for the first time in top drawer - the unit does not cut off for about three hours!!
It is still under the initial 90 day replacement guarantee. is this the norm for F/Fs or do you think i have a duff one?

Thanks a lot

Confused Pensioner

Gordon