Loud beep coming from a brown box near the floor

Keep in mind that when you replace the detector, it's hard wired into your 12v system.  You'll need to cut the wires to install the replacement.  Be sure to pull the fuse to the detector or turn off power in the 12v system first (that includes disconnecting the AC if it's connected).  Also, note the original installation may not follow the red/black DC wiring convention that you'll see on the replacement detector.  One easy way not to get confused or get positive/negative mixed up is to cut the wires at where they exit the old unit and use some compression connectors to connect the new detector to the old wires.  I used spade connectors so the next time I have to change the detector, it'll be just a simple task of unplugging them, installing the corresponding spade connector on the new unit, and plugging it in.  The manufacturer makes a hard connection to avoid people from easily disconnecting detector so they don't kill themselves by eliminating the warning sound and light in the event of a leak.  I think most people are sensible enough not to do that if they have replaced the unit using the method I described.