To answer your question it's very easy to remove the center speaker. Just take out your license or credit card and place it in the seam between the grill and dash. Gently lift the grill and it will unsnap. Once you remove it then get a small socket out and remove the two bolts holding the speaker. Then unclip it from the harness after you lift it outside of the area. Voila!

Hope that helps!

Vintage

 

Thanks Vintage, this is exactly what I wanted to know!
I'm at the office now (I live in France / Paris) but I will let you know if I manage asap.
By the way, I'm surprised not to find any post on this item because I encountered this noise on both of my previous IS as well. It's really annoying when you drive on a bad road...

 

I just managed to lift off the grill with the help of a credit card. Tomorrow I will continue my search for the cause of the noise...