Prise the key off carefully. I think the keys have scissor jacks underneath them - it's probably those surfaces that are squeaking as they rub together. Face away from the computer and spray some WD-40 onto a cotton bud (Q-Tip for y'all Americans out there), and then gently rub the soaked cotton bud on the moving parts. Click the key top back on and you're good to go!
If the key top is rubbing against adjacent keys, you can tell by looking at the keyboard in a dim area with the backlight on, and examining the clearances around the edges. If it's rubbing, prise the key top off and gently trim the offending edge by shaving with a (very) sharp blade - craft knife, scalpel or similar. Do it a fraction at a time and take your time, you only want to remove the miniscule amount that sticks out and not go so crazy that the trim is noticable as a contrasting dark mark on your otherwise lovely silver key top. use a sharp knife otherwise it may stick and gouge too large a piece out, or worse, slip suddenly and cut you.
By the way, I accept no liabilities here. Do any or all of this at your own risk. The thing is to do it gently, carefully and take your time.
