That's as good a guess as mine Gareth. It could be almost anything from the jack bouncing around the boot to a door not latched properly. I have to say the previous diagnoses of wheels too big or dry suspension are sheer fantasy without other supporting evidence, which would be very visible and unmissable to a mechanic who knew what he was doing.
Zara. You need to get the car looked at, preferably by someone who has been recommended by someone you know and trust. Be aware, if you are not already, that some garages will try to take advantage of women who have no knowledge of cars and tell them any old rubbish to defraud them. That applies to both big and small businesses so try for a personal recommendation, not an internet review site.