[Saving-A-Dime]

When accelerating I always hear this knocking sound in the back of the car. Hoping someone can help where I should look to identify the sound as BMW dealership could not. It generally follows a pattern of Knock Knock, pause...., Knock. I have scoured YouTube for anything similar and I haven't found anything. Also, as I get faster, a continuous noise like road noise seems excessively loud in the rear, with faster I go the louder it becomes. Could this be something with muffler? Thanks