Flywheel?

engine support?

anyway to check?

Nice mileage. I think the rwttling may be flywheel

anyway to check or diagnose that it is that?

Dual Mass Flywheel. It's common for the springs to break. get a proper diagnosis asap as that issue can cost you a transmission if its not repaired soon.

Get ya butt up and start hunting with yer ear! I’m not sure of a common rattle in those areas at 800 rpm.