Does it happen with all or just some CD's? If it's all, then I would get it checked out, either by going to Apple's support page, calling them, or sending it in... If it's just those CD's, they may be off balance, but I've never heard of that being a problem, especially since the iMac's CD drive is a tray-loading drive...
Of course, your drive may just be operating normally, it just spins so fast, that an unperfect CD that's just a bit off balance may seem to make the whole thing shake, but I don't think it should be too excessive... I had the same problem in my old 300mhz blueberry iBook... The CD drive eventually died, and I had to send it in to get it replaced, and the new drive was just as noisy...