I had been hearing a clicking sound that I thought was the side mazda logo coming loose and just vibrating against the body...but I found that I couldn't have been farther off.

I popped the hood and noticed sparks jumping from one of the plugs on the battery...that's what was making the noise...

Then I looked down to the lower two plugs on my 12A engine, and they were disconnected! This is probably why my tachometer was going crazy...

So does this mean I've been running with half an engine since like, 5/2 ???

That's insane...this damn thing's TOUGH!

 

when the wires that should've been connected to your lower plugs were hitting the frame or engine and grounding out, you'd hear something that sounded like a pop...

Enough pops could sound like a rattling I guess, but either way, it certainly couldn't have been good for your electrical system

 

You weren't running on half the engine, since both plugs on a single rotor fire in the same chamber, but you were probably getting a lot of incomplete combustion (and hence, lower power, and worse gas mileage). Those bottom two plugs are probably so gunked up at this point, that you should just go ahead and replace all four, as well. Likewise, new battery terminals sound like they're in order, and they're really cheap ($3 i think).