Replacing factory speakers in a BMW 740i with MB Quarts 

Here I try to explain how BMW went through great lengths to design sealed, tuned enclosures for their door speakers, and how you can end up with horrible results by just swapping them out with even the most expensive of speakers.
In order for these MB Quart speakers to sound halfway decent and not buzz, you have to drill holes with a hole saw, all around the plastic enclosure so that it's vented to the air in the door. You could also just cut out sections with a rotary tool or whatever, -so long as you open it up to the air inside the door.