I noted my problem above but since a couple months ago the problem has gone away on its own. I think it's because the tires aren't brand new anymore. Nothing else has changed... well maybe except some tire pressure (measured low 30s the other day and I have yet to put more air in).

vibrating front seat
 

Looks like I eventually sorted my seat "vibration" problem. Right under the middle of the front passenger comfort seat (you have to move the seat forward and get your head right under the back) theres a grey cable with a blue connector. Its cable tied to a white plastic brace across two seat springs, I basically undone the connector and cable tie then followed the cable back under the leather to where it disappears up the seat. You can feel its touching the metal frame and this is where the problem was for me. I basically re-routed this cable in a different direction and all seems well. This has been driving me crazy since I got the car but it didn't always make the noise. Recently though with colder weather this has made it worse so I'm glad its sorted. Its wierd now driving with silence (well at least some silence) still got some creaks and groans from the rear.

I have found the solution.
 

Hi guys,
I have located the source of creaking sound.

Please refer to my drawing. The black circles stands for the joints (or hinge), the red lines are leather of seat. When vibration occurs, the leather rubs the joints then cause the creaking noise. (Indicated by light blue curve.)

So, my solution is to insert a 1.5 c/m rubber foam inbetween the back rest and seat cushion to force the leather to be sticked onto the joints.
You just need to put inserts on both corners. (I make 2 red circles on the photo.)

Also, you can try to isolate the leather from touching the joints on the light blue area. I have tried before but did not solve the problem totally.

I hope my solution can help. If you have any other idea, please share with us.


ps. The photo was "borrowed" from BMW web site. I hope BMW won't sue me for copy right.