thump

Could someone identify this for me, I’m hearing an annoying clicking thump sound every second even with the music off.

So...I am building a new box for my truck I removed bench and made a olds 3rd row seat fit..Now I have a large area to build a good box for my 2 massive audio kilo 104s..Normaly I build a Box to fit where ever I can..This is my first box like this so I'm going to over build the **** out it...

Let me begin with, I am happy to be a part of the community, and glad I can provide service at time. This is my first post, and am just now starting to get into an audio system. Just bought a new car, and along with that a system, and am trying to accustom myself accordingly. Thanks and I...