Have not Experience that but have heard anything like this
The noise is prevalent when the car is cruising approx at 45 MPH in 4th gear, 0r 35 mph in 3rd gear when you let off the throttle and the car is in deacceleration mode you hear and feel a clunk and then when you get on the throttle and the car gets back into acceleration you hear it again. I believe it might be the driveline connection to the transmission, possibly the U-Joint. Has anybody else experience this.
Also they Vehicle has had an engine Swap covered under warranty and BMW certified Dealership, Prior to the engine swap the noise was not present.