I own a 2005 BMW E46 320D. recently i am hearing a loud knock noise just below and back a few inches from the drivers armrest it usually only happens when im starting the car and then its gone, but if i cause the car to slide i.e. doing donuts ,it starts rattleing bad and stays like that until i turn the car off and on again where it knocks once and stops, does anyone have any ideas Thanks

Jack the car up and look under the car where the noise is comming from and I think you will see where the exhaust pipe could be hitting someething.Get a pry bar and just move it back into possition. I love donuts,I used to do them all the time when younger.

whatever it is it's probably damage due to the donuts - could be loose exhaust mounting.

it could be your caltilatic converter, the one on my fathers cadilac make noises like your discribing