Hm.. Those springs are in their pretty tight. But I guess the only thing to do now is take it apart and take a look. But then again yours is fairly new right? Not sure if taking it apart voids the warranty but I bet it does. So maybe give elitekeyboards a call and see what they have to say about it, assuming of course that is where you got it. I have to say this is the first time I've heard of one making a spring type of noise.
Wonder if it is sticking or something and what you are hearing is it coming unstuck such that the key is poping back up faster than the spring so that it rattles a little. Maybe give it a little time to work out before calling elite then. I think ripster mentioned something about finding grease under some of the domes of his 87U when he took it apart. When I pulled apart my HHKB Pro I did not pull the domes up other than the one on the space bar and it was clean.
EDIT: I guess my hearing is not as good as it used to be but ripster is right I just tried what ripster suggested and yep hear it. Maybe I just filtered out that sound because I expect a board with a spring to ping every now and then.