Hey mate,
I had the same problem yesterday I initially thought it was from the cabin/boot space as well. After a week of pinpointing the issue, I found it to be the mounting on the exhaust near the tip... there seems to be a spring mechanism that locks it into place, and when it came loose, made a rattle that sounded like it was in the cabin space/panel work. I'm not sure if that is the same case for you, but I guess you could check...
Thats the case for my R34, but I think it may be the same for you as well.
Good luck~!