Sue, ask your friend to go back to the Garage and DEMAND the used parts back ( I have done this for a friend a few years back) then Contac BMW UK, ask your friend to ask BMW to look into this case and the attitude of the Manager, the cat would not make this sound, going off this so called Knock, that could b down to several things, Shocks, Wheel nuts loose, something under the seats ( Yes I have heard that, as it happens it was a tin of peas) soething in the boot, spare wheel loose in the tray, I would advise that your friend stays well clear of this garage as they don't seem to have a clue, & that for a BMW Garage is very bad, as said, if you can not get any satisfaction call T.S.