Whats wrong with my bmw x5 ?

i have a 02 bmw x5 4.4i , whenever i cruise for like 45 mins continous i start feeling that when accelerating then pressing the brakes till a stop i feel a bump in the car as if somthin has hit it , sometimes its strong and sometimes barely felt ? what is this ? a gear problem or a torque convertor one ? is it serious ? there r no alarms or msgs that have appeared on the car's computer ? so what might it be ?

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sounds like cv joints

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Rear subframe rubber mount maybe worn out. When you accelerate the car, the subframe mount will stretch, and when you slow down even for just little bit, the stretched mounts will send the rear axle to up direction, causing it to hit the car's frame.

How hard you get hit is based on how far the subframe rubber mount stretched down direction during acceleration process, and the problem is more visible in low rpm (0 - 30 miles range) Car rear axle uses H shaped frame, so you have 4 of them and parts would run around $50 each. Removing them will require some type of pressing tool so I would service this in the dealer.
it is probably a programming issue. take it to your local dealer and have them check faults. if none are present, then they will probably flash your egs(transmission module).

It can also be your driveshaft coupler. it can cause a jolt during load reversal or acceleration.
Possibly universal joints on drive shaft.

CV joints possibly if it occurs while turning the steering wheel when it happens.
its time for the new 2007 x5