Ive got the same problem with my 2.2 Veccy, did you get it fixed, what was it in the end? I had vauxhall look at it, they replaced the cat, flexipipe and centre section, but it still bloody does it!!
Its sounds to me like its coming from somewhere in the engine, so perhaps its the manifold, but vaux should have picked it up?!?
Had similar sound on a Mondeo and that was tapits.
HELP!!!!!!