I've just had a 'After Market' full exhaust system fitted to my 406 (diesel) . With the new exhaust at certain revs when pulling the car vibrates / rattles and it feel more like a transit van than an exec car.
Personally I think this is duff exhaust and I'm asking the garage to fit a replacement ASAP. Is this a problem with non pug exhausts? I've never had a problem with 'After Market' parts in the past. Garage also said that it has seen this problem with later vauxhalls and a 306.