Engine not starting / and other weird symptoms

Hi there, the truck in question hadn't been started in two or
three weeks because we haven't gone anywhere. Here's what
happened when I tried to start it two days ago.

1) It sounded like the engine was trying to turn over but
couldn't. All the interior and dash lights were on, so I didn't
think it was the battery. But I hooked up a small battery
charger to it anyway, and according to the battery charger, the
battery is fully charged. I also didn't see any obvious
corrosion around the terminals or anything like that. I also
checked that there is oil, and gas.

2) When I turned the ignition, there was a loud rapid clicking
noise in/near the ignition. Then after a few tries, I was only
getting the clicking noises (no more sounds of the engine
trying to turn over).

3) While sitting in the car with the key only turned halfway,
several other strange things happened. First, the stereo went
through some sort of factory shut down process, which made
me think something electrical was going on. Then I heard a
quiet sound near the floor, underneath where the
stereo/AC/heat controls/etc are -- sort of like the sound of a
phone vibrating. It occurred off and on every couple minutes.

4) Then the dash light came on that says "SERVICE BRAKE
SYSTEM", which I have never seen before.

5) Then I started hearing a slow steady clicking sound
underneath the driver seat. When I looked underneath the
vehicle in that area, it appeared to (maybe) be coming from
one of the two lines that are running (from front to back) just
underneath the driver seat area. This sound persisted until I
turned the ignition completely off.

I have left it alone since then! Thanks!