The cover on Sparco racing shells is held on by a wire with a spring to keep tension in the wire. As the wire loosens up, the spring starts to rattle against the fiberglass/CF, and it sounds exactly like the rattle is coming from under the dash. It took me awhile to hunt this one down.

I fixed it by getting some self adhesive velcro, about 1" x 2", and sticking the loop side to the shell right under the spring.

If you're putting in shells, I'd just do this before you even install them because it's hard to stick on the velcro with the seat in the car, and my guess is that spring is going to rattle eventually on every shell.