Metal - I originally had the cavity boom with my sunroof and then read about the "blue tape". After removing the blue tape, the cavity boom was gone. I then installed the sunroof wind deflector and the cavity boom is back. Since I have the window rain deflectors, I just slightly open the rear windows and the cavity boom is gone. I like the way the sunroof deflector looks but I'm not sure if I'll keep it on at this point or not...