Mine is a confirmed heat shield rattle. Got my auto teacher to look at it, didn't even have to think about it, heat shield rattle. Mine sounds internal as well.. sounds like it is coming right from the flywheel too. Check your heatshields wj.... if any of them are loose, you should consider replacing the rivits. PITA because you have to pull them off....
The easiest way to reproduce mine is going under the hood, engine running, the throttle has like the first 1/4" of movement real easy to push then past that is harder.. well push it down that 1/4" then let off, it'll make that rattle with a slight delay after the decel starts. Under no load btw.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.