White plugs on subwoofer + rattling subwoofer
I have two related questions:
1) I am trying to remove my Harmon Kardon subwoofer so that I can try a fix out that I found in my BMW repair manual (see part 2 of this post). As per the instructions, I have removed the 4 screws, but there are these two plastic things that seem to be holding it in. I cannot figure out how to remove them, without destroying them. I have attached a picture. Does anybody know how to take these off? I have tried to spin them...and that is pretty much all I can think of. It seems like these are the only things holding it in.
2) When playing certain songs, that are have some bass, there is a noticeable vibration noise where the sub-woofer is. Looking carefully, it looks like the issue isn't the speaker, but the plastic on top of the speaker. Looking from above, it's right near the clip that can be used to clamp down a baby seat. I can actually see the plastic vibrating up and down and when I put my hand on it, the noise goes away. Does anybody know a solution to this?
-I have tried to tighten every screw I can see (which is not many), no luck.
-I have tried to shove in some old foam mattress into the holes around the subwoofer that I can see, hoping to stop it from moving as much. I haven't had time to really test out how effective this is, but I think this will only help slightly since it's the actual plastic above the sub-woofer. Obviously, I don't want to put anything between the plastic and the actual speaker.
-I bought the official BMW repair manual. It describes my problem reasonably closely: "In audio mode, creaking and rattling can be heard from the area of the subwoofer". It describes removing the subwoofer and "gluing a felt strip on the crossmember in the area of each of the subwoofer's two front clamp connections". I want to try this out, but can't get the subwoofer out safely (see point 1).
Thanks in advance for any help.
My car is a 2001 BMW 330ci with Sport\Premium option, plus the Harmon Kardon surround sound option.