What causes this? And anyone know how to fix it? It's annoying hearing rattling all the time while driving....

 

Well it could be a window, a speaker, window regulator or motor. A little more help we might be able to figure it out.
I have a rattle in the drivers seat. It my brain rolling around in my head. Ha Ha

 

Could also be a hinge bushing that's toasted. Open the door & grab it from the bottom & lift up. Try this with the door in different positions of 'openess'. If you can lift it, get a replacement bushing/pin set ASAP or you run the risk of rounding out the hinges themselves & they're riveted on, not a fun job aligning the door when you replace them.
One odd trick I read years back was putting a small rag folded over under the hood by the hinge on the noisy side. If the rattle goes away, the helical washer (domed washer) has worn out & has to be replaced.
Mine had a noise/thunk under the PS dash... nope not in the dash but the hood hinge washer.