It's no biggie, very common, just throw a hose clamp on it. Or a lot of people just tear it off, but my opinion is if Nissan thinks there should be a heat shield there, then there should be.
Edit: after re-reading, I take it your rattle is gone but just wondering what the little rusted piece is? It's part of a hanger I believe, I pulled off a similar piece.
2002 Super Black 5 Speed 4x4
: AC Shackles and UCAs, Extended Bilsteins, 2" BL
: front ARB Bumper, rear Skid Row bumper with custom tire carrier
: Bandit 4x4 idler arm bushings, AC HD TRAs, Calmini IAB; Moog TREs and 555 centerlink
: 33x12.5x15 Goodyear Duratracs, desert runner wheels
: K&N intake, Doug Thorley headers, Magnaflow muffler
: backup camera, Pioneer head unit, JBL 770w amp, JL Audio 12" W0, rear Sony Xplods