Im adraid the information is too vague for a diagnosis,
Check the heat shield on the cat under the bonnet. This can corrode and come loose. Can sometimes be fixed with somw washers to re sopport it.
Look under the car and see if there are any broken hangers or missing rubber mounts.
Check for silencer damage as it could be a loose baffle.
Look at / near the flexible joint at the front at the botto of the front pipe.
My local tyre man is usually prepared to do a visual check free.
Check any heat shiels on silencers are not loose. If so fix straight away!!