My wifes Astra-G Z22SE has got some strange rattles too. First cleared the car boot/glove box etc of stuff - so nothing could move while you drive. Have someone in the passenger seat and go for a drive with the windows down. Note rev and road speed and try and see if you can locate the noise at the front or the rear.
Sound #1. I found that the metal braid on the exhaust downpipe had come away over time and hence when the engine moved (ie changed gear), it would scrap on the metal down pipe. No MoT failure, just annoying sound. Will repalce when/if CAT needs to be replaced. So for now I'll just put up with it.
Sound #2 was similar but at the rear, the exhaust back box rubbers were too loose so the back box was tapping/knocking. Some cables ties later put the back box in place.
Sound #3 was a scraping sound at the rear that was the same as road speed. Found the stone guard around the rear brakes was scraping on the rear discs. Bend the guard out the way job done.
Sound #4, engine bay sounds, sometimes the engine sounds like a diesel, very tappety but soon dies down. Seems to be fanbelt/timing chain area. The engines done 110K so I guess its due for a timing/balance chain change.