Vehicle hood vibrates under both idle and driving speeds. At highway speeds or bumpy roads the vibration / flexing of the hood metal causes sun and light glare (like a strobe light effect) causing visual disturbances making it hard to stay focused on the road. There is no seal above the headlights which allows additional air under the hood — the faster the speed and / or windy conditions , the more the hood flexes and causes reflection to the drivers eyes. The easiest explanation is if the hood safety latch is engaged and you see the mood moving up and down prompting you to stall and see if the hood is loose.