Ratting noise - clutch/exhaust?

Hello -

I've got a strange rattling noise (kind of like pennies in a can), that I hear sometimes when I'm accelerating. The car has 85k miles on the stock clutch, which I know is kind of worn (it's been supercharged since 20k miles), but I don't hear any rattling when I'm engaging/disengaging the clutch. The only way I know to reliably replicate the noise is by putting it in second and coasting down the hill on the way to my house. I can't tell where the noise is coming from - middle/rear of the car - it's hard to tell, but there's usually no sound unless I'm either accelerating or engine braking downhill in gear (if I pop the clutch in it will stop).

Any idea what this could be? Any help would be appreciated.

Thanks

Just took it out to see if I could find the noise...cold, nothing but after it warmed up almost every time out of the stop sign, and every time I decelerated hard enough that it was engine braking. Also I revved it up in neutral (clutch in) and when it let off, the ratting again - is this sounding like a cat going bad?