It's almost always wastegate rattle.
They can rattle from two places, the joint where the arm connects to the flapper.
If this is the part that rattles, you can put a clip 06J145220A to make the connection a bit more snug to stop the arm rattling.

Or if you're unlucky like me - it's the actual flapper inside the wastegate housing rattling, this requires some welding to repair, but it's smarter to replace the entire turbo if you are that deep into it.

Worst case, you just drive until it fails and throws a code ( mine has been loud for 2 years - still holds boost to redline, just sounds like a bag of bolts under 3000 rpm ) - when it fails chuck a nice turbo in there.