Macbook Not Sleeping, insomnia?

[MacBurg]

Hi, Ive had my macbook pro 2.33 for 2 months now and so far no problems. I usually run the mac all day, then shutdown overnight, but I recently tryed putting it to sleep. After selecting sleep from the top left apple menu, all goes well and the laptop winds down, then the screen goes black, however about 10 seconds later hard drive makes a zzzz zzzz sound and the laptop wakes up again. I have no programs running when trying to do this, and it happens regardless of whether I select sleep from the menu or use hot keys.

Also if I leave the computer running, then shut the lid, the computer will be quite for 10 seconds, then every 5 seconds from then on it will go zzzz zzzz repeatedly, very strange, and Im guessing not good for it.

Anyone have any ideas why my macbook pro doesnt want to go to sleep?