BMW Repair: 325xi exhaust system, vacuum source, butterfly valve

Question
the butterfly valve near the end of the exhaust pipe (2002 325xi)makes loud noise at start up, noise disappears after a few minutes. What is its function?Is repair necessary?  What are the options?

Answer
I believe that valve is vacuum regulated.  Remove the vacuum source and see if the problem persists.  If it remains, you may need a new valve.
This valve is used when the engine load increases; It bypasses the muffler in order to reduce the exaust back pressure (giving you more power)
-cheers