Ulysse exhaust leak

Hi all, I have a 2003 Ulysse Eleganza with 105K mile and is leaking gas/exhaust/vapour whatever form the top of the engine at the very back. We have taken it to a couple of garages (including local dealer in Belfast) and suggestions from oil return pipe disconnected or broken to the injector. Vehicle seems to drive okay, no smoke belching form the exhaust except on start up but the exhaust noise is in time with the revs of the engine. Anyone else come accross this and is it expensive to repair? Thank you for your anticipated reply