Because it's just two pieces of plastic held together with a bunch of plastic pins. There aren't enough pins to hold it truly solid, so when the pins move and or the two halves touch each other, you get the characteristic rattle/squeak/crack of the privacy cover.

I'm posting a few pictures of the cover for those who are interested of what it looks like disassembled. I plan to reassemble it with both tape around the edge where the two pieces meet and a liberal amount of silicone in the hopes it will form one solid piece.

If that fails, I'll ask my dealer to order a new one, but unless they changed the design I don't see how that would solve anything...

Thanks again to Denny, Lloyd and Jef Sessler (hey, my dorm "house" in college was called "Sessler") for the fix and encouragement...