Vauxhall Corsa graunching sound!

Had my car for all of a week, when I went out for the test drive, I did not hear this graunching sound. The salesman was beside me in the passenger seat. Now, I have started to drive and get out on my own, only to hear this graunching sound coming from the rear nearside, doesn't happen when I am on a smooth road, but going over bumps and uneven road, it is very irritating and distracting. Have checked that nothing internal seems to be loose, and that seat belts, and everything in the boot is secured. Has anybody else heard of this, and whether this is something serious, will inform sales manager tomorrow.