recoil. Got any pics?
A properly designed and fitted exhaust system shouldn't contact any part of the chassis/bodywork.
As I'm sure you are aware it has to be independantly mounted to allow the system to expand and contract during use,it's also to reduce noise transmission to the body.
Sounds to me it requires a little 'tweaking' to get right,it might also be due to soft rubber mounts.
Are your engine/trans mounts in good order?,excessive drivetrain movement can also exacerbate this.
In a way you've almost answered your own question,but it might be an idea to try another shop.
To "expect some clanging" sounds like your present shop are not interested in solving your dilemma.JMO
