Rattling sound between engine and trans

Of course not. I was being facetious. You seem to have a lack of understanding or want to pull it apart. The flex plate is almost certainly 100% what the noise is, yet you seem to not believe it. So keep drivin it. You'll sweep it up off the road waitin for the tow truck. THen you'll know. lol
Language barrier maybe, Netherlands. If the bolts are tight and the holes not worn oversized. The 904 has a bit of history of breaking the lugs on the pump drive. I would think that wouldn’t sound like a rattle, but it might. Is this the original setup for the car or did someone install it?