Rattling sound coming from sub box

Just put my new Sundown E12 in a vented B Box There is a rattling sound when they bass hits hard. Even hear it on lower volumes. Set the sub on the ground and its def in the box. I took the sub out and it didnt rattle in my hand. I have one of the wires taped to the sub so I don't think its hitting it but wires going to the terminal cup are loose but pretty short. I could barely hook them up to the sub in the box. I also checked the terminal cup and its not that.

I took the sub out and laid it on my trunk and could hear the rattle but when I picked it up it stopped. So it's like the air coming from the back of the sub hitting a surface is creating a rattle. Just tried covering the port with a thick jacket and the rattle kind of died down.

Gain is set for 500w/13.6v so I know it's not putting to much power to the sub rated at 500RMS.