When I last had a badly rattling exhaust unit on a car it was simply down to the failure of a rubber component holding it in place. A replacement part cost less than £3 and my local garage fitted it for free. So you might well be fussing about (almost) nothing.
'Warranties' (and 'guarantees') aren't part of your statutory rights; they're a 'gift' from the manufacturer, retailer or installer. As such, the firm providing the warranty/guarantee is free to determine the conditions that apply. (It's perfectly legal for a firm to offer a warranty with a condition that requires you to take the car to their depot in Tokyo at 3am on February 29th - while singing the Albanian National Anthem backwards - in order to get a problem resolved. So they can equally include a condition that a warranty is 'non-transferable', as most warranty providers normally do).