[shlai]

It was neither DSC nor DTC holding it back. I drove an old E85 then a E89 in total of 6 years to know how it felt when DSC intervened. Although I may sound soft in writing, I am a nut when driving my Zee. I push it so hard that DSC kick in quite often. Here in Hong Kong, it rains quite often in summer and my way home is steep slope up, and my rear wheels slip often.
The golf tee definitely made the car sounded deeper at low RPM, especially at starting up the car. Even my wife knows nothing about car but has very good musical ears, she could hear the louder, deeper exhaust - I didnt tell her in advance.
What I meant was I believed the exhaust flap was not functioning properly. For the last few days, without looking at your reply, I connected back the hose to the acutator to reactivate the flap, and the old problems came back. BMW should have designed the flap more sophisticatedly, for example, bearings to the rotating axis of the flap.