[meltawab]

I have a problem in my BMW and I’m looking for some thoughts, I will divide it into sections for better reading.

About the Car:
BMW 2009 sedan 328i XDrive (AWD) - Automatic
Mileage: 102000

Problem:
I feel a vibration that comes from below/under the car that feels/sounds like the car is driving over rumble strips when accelerating, when this happens the car is harder to accelerate.

Conditions/comments about the problem:
1. After driving the car for 20-30 minutes (never happened directly at cold start regardless of my driving behavior).
2. When accelerating (push on the gas pedal), mostly on 2/3/4 shift.
3. It’s not related/tied to the car speed, it’s on accelerating.
4. No Errors or Check Engine Light.

Things that tried so far to fix the problem:
1. All tires are balanced.
2. Changed engine oil and Sparkplugs (Part of maintenance, I doubt it’s related to the engine but maybe).
3. Changed the Transfer-Case Fluid and reset its adaptions (I saw a post here related to the same problem that was suggesting to start with that).
4. Reset throttle body adaptation.


What’s next:
From my research, it looks like it’s a transmission problem. Some posts suggesting to change the transmission fluid and reset the transmission adaptions however, I’m still not sure yet and I'm not sure if transmission fluid was changed before or not as I'm the second owner.


Questions:
1. Any thoughts about the problem?
2. I heard BMW transmission fluid is a lifetime! And it’s NOT good to change the transmission fluid at this mileage at all as it may cause the problem to increase, or even transmission slipping, any thoughts about that?
3. What’s the difference between changing and flushing the fluid?

Any thoughts would be very appreciated, I’m not excluding other problems such as driveshaft but I feel it’s really closer to transmission problem.

Thanks everyone!