According to the OP's last comment, the rattle was due to a broken crankshaft.
I will add one thing though. When I was a kid, my father had a '91 Mitsubishi Pajero that suddenly had its crankshaft snap in two. He was able to drive it to the dealership with a rattling noise that was present regardless of clutch position or RPM.
With regards to the OP's case, I am quite frankly surprised that the rattling noise was present only with the clutch pressed in. Not saying that it isn't possible, just unusual that the OP was able to drive "normally" otherwise.