Noisy Braemar air conditioner

I don't know much about air conditioners but with the recent heat and projections of a hot summer i am contemplating getting one. A couple of townhouses have been built 18 months ago approx. 40 to 50 metres from us. They both have identical air conditioning units which are mounted on the roofs but one unit is very noisy, how can this be? Can see that the brand is Braemer, the air con units must be fairly currrent. The air con units are cube shaped and measure about 600 to 700 mm across each dimension.