Another possibility is the engine mount. A number of posts in the forum note the poor quality of the engine mounts, especially the 'hidden' mount. Over time, the rubber wears out/dries out, and then the bolt for the engine starts rubbing on the mount. It is usually more noticeable at certain engine speeds, due to the vibration being more likely to cause noise at those certain speeds. Thankfully, there is little danger in this situation. It is simply noisy and annoying.