My system is nothing like a standard system. The flanges themselves are loose 2 bolt fitment without springs. The actual joint has a cone from the downpipe to a flared pipe at the centre section back. As for the noise, it just sounds awful when its ticking over. You can drive it in just the right rev range and you don't notice it but once you open it up it is really raspy from under the car.
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Oh, I forgot, as you said Mike, the Vectra backbox is a very similar setup but it has a tapered wire gasket that seems like a good idea.