It's something Apple does cure if you demo it. Here's the home fix.
http://www.joshoakhurst.com/?p=231
Sorry but I didn't find the all too similar article where they put the business cards in just the right spot under the battery to tighten up the mouse pad/button.
The plastic noise you write of is not unknown but the real fix is at a genius bar and getting a demo of it so they see what to fix.
Bob
Problem Computer: 3 month old Macbook
In the past few days I have discovered a very annoying problem with my macbook. The trackpad button (is that what it is called?) seems to be loose, or have some kind of problem. Specifically it's on the left side of the button. When I click it, it makes a clicking sound like a cheap mouse would make, then I push more and it clicks. The sound also reminds me of the sound my teeth make when I click them together, except maybe the trackpad button is more high pitched.
Either way it's really annoying, and I would like it to stop as soon as I could. I do have Applecare, but sending this thing away wouldn't be fun at all.
Any idea what it could be or how I could fix it?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic