uMB Vibration Problem

Hi,

I know it might be nothing, but just thought I'd make sure. I've been using an Aluminum Macbook for a few months, and it's been working super great, well, until yesterday, I'm not even sure if it's been this way the whole time, but it's been driving me nuts since this morning. The problem is, I've been feeling a little vibration around the entire trackpad area, especially on the right side, it's probably coming from the HDD (Factory drive 160GB), but I really don't like it. It's nothing noticeable, I mean you can't tell the difference until I've told you about it or unless you've been resting your hands on it for a while, but the point is, it does exist. Now my question is- is it normal? Was it there the whole time? If it's not normal, should I be taking it to the store? I've never even opened up the bottom, ever, it's been as it was on day 1. I tried listening to it carefully while giving it a little shake but nothing seem to out of place in there, it still feels as solid as new, but the bottom line is, I'm going crazy. What should I do?

P.S. It's got nothing to do with electricity cuz the vibration remains when I run it on battery.