Ok so hopefully this one we can pu to rest
Here's some background info on how the magic exhaust valve works
The exhaust flap reduces the noise level when the engine is idle and when the engine speed is close to idle. This reduces the outlet noise to improve comfort for the customer.
The reduction in outlet noise is appr 8db .
The engine control activates the exhaust valve.
The degree of opening depends on gear, engine load and engine speed (see chart below).
Only two conditions , open and closed, occur while driving.
1 Gear Engaged
2 Exhaust flap , closed
3 Exhaust flap, closed
4 Data point for engine speed
If the exhaust flap is faulty (or tempered with) the exhaust system will emit a hissing noise at high engine speeds when it's closed.
If the exhaust flap is defective and partially open a humming noise will be generated at idle.